Legion II. ACANTHARIA,

vel Actipylea, vel Acanthometrea (Pls. 129-140).

Definition.—Radiolaria with simple membrane bounding the central capsule, which is everywhere perforated by innumerable fine pores (disposed either equally or symmetrically). Extracapsulum without phæodium. Skeleton centrogenous (its growth proceeding from the centre), acanthinic (organic, not siliceous). Fundamental form originally spherical.

The legion Acantharia vel Actipylea, to the extent here defined, was constituted by me, 1878, in my Protistenreich (p. 102) under the name "Panacantha." A more accurate definition of this group was given in 1879 by Hertwig under the name Acanthometrea. Both names were replaced by me, 1881, in my Prodromus (pp. 421, 465) by the more convenient name Acantharia. This legion comprises all those Radiolaria which were first described by Johannes Müller, 1858, as Acanthometrae, and also an important part of his Haliomma. In my Monograph (1862, pp. 371-424) I disposed them in three families, Acanthometrida, Diploconida, and Dorataspida.

Although the number of genera and species in this legion is much increased by the rich collection of the Challenger, we can divide all Acantharia into two different orders: Acanthometra (without complete lattice-shell) and Acanthophracta (provided with a complete lattice-shell).

The Acantharia agree with the Spumellaria in the structure of the simple capsule-membrane, which is perforated by numerous small pores (but constantly devoid of the large main opening, which the Nassellaria and Phæodaria possess, being hence united as "Merotrypasta"). We can therefore unite both former legions as "Holotrypasta" (compare above, pp. 5, 6); but in many Acantharia (if not in all?) the numerous small pores of the capsule-membrane exhibit a certain peculiar arrangement not observed in the Spumellaria; therefore the latter can be regarded as true "Peripylea" in opposition to the former as "Actipylea."

The peculiar main character of all Actipylea or Acantharia is determined by the chemical constitution of their skeleton, which is not silex, but a peculiar organic substance, called by me in 1862 "acanthin" (Monogr. d. Radiol., pp. 30, 32). In all other Radiolaria the skeleton is composed of silex or of a silicate. But besides this chemical difference, an important morphological character of the skeleton also separates the Acantharia from all other Radiolaria: in the latter the skeleton is never centrogenous or arising from the centre of the capsule; in strict opposition to this general fact the skeleton of all Acantharia is centrogenous, composed of radial spines, which arise from the central point of the capsule and pierce its membrane. These characteristic "radial spines of acanthin," arising from the centre, are never hollow (as formerly was supposed), but constantly solid. Their form is extremely variable, and most important for the distinction of genera and species; but more interesting from a general point of view is their peculiar arrangement or disposition.

The regular disposition of twenty radial spines has general value almost for all Acantharia, with the exception only of the small group of Actinelida. In this latter group the number of radial spines is either more or less than twenty, and their disposition is either quite irregular or follows a peculiar rule. The number of individuals of these Actinelida, compared with that of the other Acantharia, may be scarcely 1 per cent., whilst the latter have more than 99 per cent.; the number of observed species is in the former about 5 per cent., in the latter about 95 per cent. Nevertheless the small group of Actinelida is very important, being probably the ancestral group from which all other Acantharia have been phylogenetically derived. These other Acantharia, with twenty regularly disposed radial spines, represent the two large groups of Acanthonida and Acanthophracta. For short and clear distinction of these two groups of Acantharia, we will call the Actinelida (with irregular number and disposition of radial spines) Adelacantha, in opposition to the Icosacantha (Acanthonida and Acanthophracta), which all possess twenty regularly disposed radial spines.

Johannes Müller, the great zoologist, to whom we are indebted for the first detection and accurate knowledge of the Acanthometra, already recognised the regularity in the peculiar disposition of their twenty radial spines (Abhandl. d. k. Akad. d. Wiss. Berlin, 1858, pp. 12, 37). In honour of my great master I have called this regular disposition the "Müllerian law of spine disposition," and have given a full explanation of it in my Monograph (1862, pp. 40-45, 371, 372). With regard to its general value for all Icosacantha (Acanthonida and Acanthophracta), we might also call this promorphological Müllerian law "the Icosacanthan law."

In 1862 I had already given the following precise definition of this "Icosacanthan law" (loc. cit., p. 40):—"Between two poles of a spineless axis are regularly disposed five parallel zones, each with four radial spines; the four spines of each zone are equidistant one from another, and also equidistant from each pole; and the four spines of each zone are so alternating with those of each neighbouring zone, that all twenty spines together lie in four meridian planes, which intersect one another at an angle of 45°." For the clear conception of this remarkable Müllerian law, and for the complete understanding of its high value for the complicated morphology of all Icosacantha, it is the most profitable way to retain constantly in mind for comparison the figure of a terrestrial globe with its axis and zones. The axis of the globe is the spineless axis of all Icosacantha, around which all twenty spines are symmetrically disposed; it is perpendicular to the bisecting equatorial plane, in which lies the middle of the five parallel zones; therefore the four spines, crossed perpendicularly in this equatorial plane, are called the equatorial spines (c1 to c4 in the figures of Pls. 131-140); often, and mainly in the family Quadrilonchida (Pl. 131), these four equatorial spines are much larger or of a peculiar form, different from that of the sixteen other spines. Each pair of the four equatorial spines lies in one equatorial axis, and this latter is perpendicular to the crossing axis, in which lies the other pair of opposite spines. We may regard these two equatorial diameters, perpendicular one to another and to the spineless axis, as the two perradial axes or primary axes. Correspondingly the two meridian planes, which are determined by one perradial axis and the spineless axis, may be called the two primary or perradial meridian planes.

The globe is divided by the equatorial plane into two equal halves, the northern and the southern hemisphere. In each hemisphere there are disposed quite symmetrically eight radial spines, the distal ends of which fall in two parallel circles, a larger tropical circle (nearer to the equator) and a smaller polar circle (nearer to the pole of the spineless axis). Therefore we call the four spines of the former the "tropical spines" and the four spines of the latter the "polar spines." The angle between the former and the equatorial plane is about 30°, the angle between the latter and that plane about 60°.

The eight polar spines (four northern and four southern) lie in the same two meridian planes as the four equatorial spines. Therefore in each of these two perradial planes lie six radial spines, opposite in pairs; two equatorial and four polar spines. Commonly all eight polar spines are of the same size and form; and often they are also equal to the eight tropical spines; but in some cases (e.g., in some species of Quadrilonchida) they are much smaller than the twelve other spines, and sometimes even rudimentary. In all figures of the Pls. 131-140 (and also in my Monograph, 1862, Taf. xv.-xxii.) the polar spines of the northern circle are marked by the characters a1 to a4, the polar spines of the southern circle by the characters e1 to e4. In the first perradial meridian plane lie a1 and a3, e1 and e3, in the second a2 and a4, e2 and e4.

The eight tropical spines lie between the eight polar and the four equatorial spines, four in each hemisphere; their distal points fall in two parallel circles, which correspond exactly to the two tropics of the globe. Therefore the four northern tropical spines may be called "canceral spines" (as their ends fall in the Tropic of Cancer) and the four southern correspondingly "capricornal spines" (as their points lie in the Tropic of the Capricorn). In the figures of the Pls. 131-140 (as well as in my Monograph, 1862, Taf. xv.-xxii.) the four northern or canceral spines are marked by the characters b1 to b4, and the four southern or capricornal spines by the characters d1 to d4. Also the eight tropical spines lie (crossed in pairs) in two meridian planes; they do not lie, however, in those perradial planes, in which are placed the twelve other spines; but in two different meridian planes, crossing the former at angles of 45°; we call these the "secondary" or "interradial" meridian planes. Each of these planes is determined by the spineless axis and by two crossed interradial or secondary axes; in each of the latter lie two opposite tropical spines. In the first interradial meridian plane lie b1 and b3, d1 and d3, in the second b2 and b4, d2 and d4.

It is a most interesting and important fact, that in all Icosacantha (Acanthonida and Acanthophracta) this regular disposition of the twenty spines (in five parallel zones and four meridian planes) becomes constantly preserved by heredity, whilst the form and size of the different spines are extremely varied by adaptation.

Only in a minority of the Icosacantha are all twenty spines perfectly equal or nearly equal in size and form; and then it is often very difficult to distinguish the different zones in their disposition. But in far the greater part the size or the form of the twenty spines becomes different in different zones; and then we can commonly distinguish easily the five different zones. Firstly, in all Quadrilonchida and Dorataspida, the four equatorial are distinguished from the sixteen other spines either by form or by size, and often in a very remarkable degree. As soon as these four principal spines are recognised, it is easy to determine also the sixteen others; for the eight polar spines lie in the same two (perradial) meridian planes as the former, whilst the eight tropical spines lie in two different (interradial) meridian planes, intersecting the two former at angles of 45°. Commonly, therefore, this distinction is rather easy.

In the majority of the Icosacantha all four equatorial spines are exactly of the same form and size. But in four families the two opposite spines of one equatorial axis are much larger, or of another form, than those of the crossing axis. This is the case in the Amphilonchida, Belonaspida, Hexalaspida, and Diploconida. Therefore we here call the major equatorial axis (with larger spines) the "hydrotomical axis," and the minor axis (with smaller spines) the "geotomical axis." Correspondingly, the meridian plane, in which the two larger equatorial spines are placed (c1, c3) and the appertaining four polar spines (a1, a3, e1, e3) may be called the "hydrotomical plane"; in the remarkable family of Hexalaspida (Pl. 139) all six spines of this hydrotomical plane are much larger than the other fourteen. Perpendicular to this plane is the second perradial meridian plane, which we call the "geotomical plane"; in it lie the two smaller equatorial spines (c2, c4) and the corresponding four polar spines (a2, a4, e2, e4). In some Hexalaspida (Hexonaspis and Hexacolpus) the six spines of the hydrotomical plane become so preponderant that the other fourteen spines appear rudimentary; and in some of them the two equatorial spines of the hydrotomical plane are much larger than the four polar spines of the same plane. This curious relation reaches its maximum in the Diploconida (Pl. 140).

The different development of the two equatorial axes (of the larger hydrotomical and the smaller geotomical axis) is the first and most important cause of the peculiar forms, which are produced in the four cited families. We derive these terms also from the metaphor of the terrestrial globe. The hydrotomical plane is that meridian plane of the globe which intersects almost only the water-hemisphere (the island of Ferro in the Atlantic, the island of Pandora in the Pacific). Perpendicular to this is the geotomical plane, the meridian of which intersects great land-masses in both hemispheres (Bombay in India, Athabasca in Canada). Both poles of the smaller geotomical axis are everywhere equal (the East Indian and the Western American). However, both poles of the larger hydrotomical axis (the eastern Atlantic and the western Pacific) are in some genera very different, e.g., in Amphibelone among the Amphilonchida, and in Zygostaurus among the Quadrilonchida. In this case we call the anterior (commonly more developed) pole of the hydrotomical axis the frontal pole, the opposite posterior (commonly smaller) the caudal pole (Pl. 131, figs. 7, 8; Pl. 132, figs. 9, 10). On both sides of these (right and left) lie symmetrically the two equal poles of the geotomical lateral axis.

The promorphology of the Acantharia demonstrates that the geometrical fundamental form in those groups is different. In the majority of the Acantharia, where the two equatorial axes are equal, that form is a double square-pyramid or a "quadrate octahedron"; the four equal equatorial spines indicate the two diagonals of the square, which is the common base of the united regular four-sided pyramids; their common axis is the spineless axis of the body; the ends of the polar spines fall on the edges of the pyramids, while the ends of the tropical spines fall on the halving lines of their faces. However, in those Acantharia in which the two equatorial axes become different, the square double pyramid becomes changed into a rhombic double pyramid; the common base of the united pyramids is thus a rhombus; the hydrotomical axis is the larger, the geotomical axis the smaller diagonal of the rhombus.

Opposed to the Icosacantha, under the name "Adelacantha," is the small group of Actinelida, in which the number and disposition of the radial spines is variable, not determined by the Müllerian law. Probably this group is the common ancestral stock, from which the Icosacantha have been derived by gradual development of their peculiar disposition. Probably the oldest and most primitive form of all Acantharia is Actinelius, in which a variable and undetermined (often very large) number of radial spines is united in one common central point, and therefore forms a needle-sphere. Whilst here all spines (often more than a hundred) are of equal size and form, in the nearly allied Astrolophus large and small spines are intermingled. Both genera together form the small ancestral family of Astrolophida. In the strange family of Litholophida the radial spines do not radiate within a spherical space (equally disposed in all directions), but within a quadrant or even an octant, forming a conical brush or pencil.

One very remarkable form of Actinelida is Actinastrum, forming the transition from these Adelacantha to the common regular Icosacantha. In the two observed species of Actinastrum we find thirty-two radial spines, twenty of which are disposed after the Müllerian law, as in the Icosacantha. The other twelve are four interradial equatorial spines (lying in the two secondary meridian planes) and eight perradial tropical spines (lying in the two primary meridian planes). Therefore here in each primary meridian plane are placed ten spines (two equatorial, four tropical, and four polar spines), whereas in each secondary meridian plane are placed six spines (two equatorial and four tropical). But here also all thirty-two spines are so regularly placed that their free distal ends fall into five parallel zones, four in each polar zone, eight in each tropical zone, and eight in the equatorial zone.

The Central Junction of the radial spines in the Acantharia becomes effected in four different ways:—(1) by simple apposition of the pyramidal central ends or bases; (2) by a basal leaf-cross, or by broad wings, four on each spine, supported one upon the other; (3) by a central concrescence of the meeting bases of all the twenty spines, growing perfectly together; and (4) by a concrescence in pairs of every two opposite spines. The most common and probably the original mode of junction is the first—by pyramidal apposition; the spines at the central base are pointed in the form of a pyramid, and the triangular faces of the neighbouring pyramids are simply placed upon one another. Often the small basal pyramids are imperfectly separated from the spines by an annular constriction. Commonly the basal pyramids of the four equatorial spines are six-sided, those of the sixteen other spines five-sided.

The second mode of junction, by a basal leaf-cross, is developed from the first and appears as a strengthening or a mechanical elaboration of it. Immediately above the basal pyramid arise from its radial edges four thin and broad triangular leaves or wings, and the meeting edges of the neighbouring wings are in apposition one with the other, so that between the bases of every three or four neighbouring spines a hollow pyramidal space remains open. The apex of such a pyramidal space is directed towards the centre of the body, but separated from it by the small basal pyramid; its open base is directed outwards. The twenty-two hollow pyramidal spaces are disposed regularly in four different groups:—(A) Four equatorial spaces, four-sided, each limited by two equatorial and two tropical spines (one canceral and one capricornal); (B) eight perizonal spaces (four northern and four southern), four-sided, each limited by one equatorial, two tropical, and one polar spine; (C) eight peripolar spaces (four northern and four southern), three-sided, each limited by one tropical and two polar spines; (D) two polar spaces (one northern and one southern), four-sided, each limited by four neighbouring polar spines.

The third mode of junction, by central concrescence of all twenty spines, was formerly regarded by me as an important peculiarity, sufficient for the separation of subfamilies and genera (Monogr. d. Radiol., 1862, pp. 399, 401; Prodromus, 1881, p. 466). But I found afterwards that in many species where the twenty spines commonly remain separated, accidentally they grow perfectly together and form one single piece of acanthin—a starrulet with twenty rays. Therefore I now think it is more natural to divide those species only into different subgenera.

A fourth and a very different mode of junction, quite sufficient for the distinction of different families, is the concrescence in pairs of every two opposite spines, lying in one diameter (in Acanthochiasma and Chiastolus). Here we obtain a number of "diametral spines" (each composed of two originally opposed radial spines) and all these diametral spines are crossed loosely near the central point of the body without any solid and permanent apposition (Chiastolida). However, in some species of this peculiar family the central part of the diametral spines is twisted like a screw or spirally convoluted (Pl. 129, figs. 2, 3).

The Form of the Radial Spines in the Acantharia is extremely varied, and constitutes the main characters for the distinction of nearly four hundred species. But all these different forms may be reduced phylogenetically to three different fundamental forms:—(a) the cylindrical (with circular transverse section), (b) the two-edged (with elliptical or lanceolate transverse section), and (c) the four-edged (with square transverse section). No doubt the first (a) is the original primitive form, from which the two others are secondarily derived. Triangular spines never occur in the Acantharia, whilst, however, they are common in the Sphærellaria. The first and original form, the cylindrical spine, is either a true cylinder of equal thickness in its whole length, or it is more or less conical. Rarely the spine is in the distal half spindle-shaped, and thicker than in the basal half. The second form, the two-edged spine, is more or less compressed from two opposite sides; its two edges are either more blunt, rounded, or more acute, sharp; its transverse section in the former case is elliptical, in the latter case lanceolate or rhomboidal. Sometimes the two edges are broader and in the form of two thin opposite wings. The two-edged spines may be occasionally shorter, triangular or lanceolate, at other times longer sword-shaped or linear. The third form, the four-edged spine, has constantly a square transverse section; the sides of this square are either even or concave; in the latter case the four edges are broadened and wing-like, but in the former case not. The quadrangular spines are either prismatic (of equal breadth throughout their whole length) or pyramidal (becoming gradually thinner towards the distal apex).

The Apex of the Radial Spines, or their free distal end, is in the majority of Acantharia simple, conical. In the minority it is either truncated or bifid, or four-sided pyramidal, often with two, rarely with four prominent parallel teeth. In some forms the bifid spines are so deeply cleft that they become forked. Much more interesting and more varied than these different forms of the distal end are those of the apophyses of the radial spines.

The Apophyses of the Radial Spines, or their "lateral transverse processes," are of the greatest importance for the morphological development of the whole subclass. Only in sixteen among the sixty-five genera of Acantharia are the apophyses perfectly wanting; in the other genera they determine in the first place their general character. In the Acanthometra the apophyses remain perfectly free, whilst in the Acanthophracta their meeting ends or branches compose the latticed shell. All differences in form and shape of the apophyses can be reduced to only two primary modes; either the spine bears two opposite or four crossed apophyses; correspondingly all Acantharia apophysaria may be divided into two different main groups, the Zygapophysica (with two opposite lateral processes) and the Staurapophysica (with four crossed lateral processes opposite in pairs). Both groups have probably no direct phylogenetic connection, but seem to be derived independently from different stocks, and produce different families. The Zygapophysica are probably derived from Astrolonchida with two-edged spines (Zygacantha), and from this group arise the Diporaspida, the ancestral group of the majority of Acanthophracta. On the other hand the Staurapophysica are probably derived from Astrolonchida with four-edged spines (Acanthonia), and from this group arise the Tessaraspida. The apophyses of the Acanthonida are partly simple, partly branched or even latticed; the apophyses of the Acanthophracta are never simple, constantly branched and commonly latticed.

The Malacoma (or the whole soft body of the Acantharia as opposed to the skeleton) exhibits some peculiarities which distinguish them from the other Radiolaria, as well in the structure of the central capsule and its nucleus as in that of the enveloping extracapsular body and the pseudopodia.

The Central Capsule is constantly spherical in the far greater number of the Acantharia, viz., in the following six families:—Astrolophida, Chiastolida, Astrolonchida, Dorataspida, Sphærocapsida, and Phractopeltida. Among these six families the Astrolonchida and Dorataspida are far greater and far richer in different forms than all the other families. The central capsule becomes ellipsoidal or cylindrical, prolonged in one axis, in the three families, Amphilonchida, Belonapsida, and Diploconida; it becomes discoidal or lenticular, by the shortening of one axis, in two families, viz., in the Quadrilonchida and Hexalaspida. Finally, the peculiar family Litholophida is distinguished by the conical form of its central capsule.

The Membrane of the central capsule in all Acantharia is simple, commonly thin, sometimes very delicate; in some species it seems to be developed late, just immediately before the formation of the spores; but in no species is it completely missing. The membrane is constantly pierced by innumerable fine pores, for the emission of the pseudopodia; but in many species (and probably more or less in all Acantharia) there is recognisable a certain regularity in the disposition of the numerous pseudopodia and of the pores by which they radiate from the capsule. Sometimes these pores are disposed in a regular network of ramified lines, whilst the meshes of this network are devoid of pores; in other cases they form regular tufts or bushes between the radial spines. Probably in no Acantharia are the pores of the capsule membrane so numerous and so equally distributed throughout as in the Spumellaria; we may therefore call the former Actipylea (in opposition to the latter, as Peripylea).

The Nucleus of the Acantharia is constantly excentric, whilst it is originally constantly central in the Spumellaria. This excentric position is a necessary consequence of the centrogenous development of the radial spines. Probably connected with this peculiarity is the other, that the nucleus assumes a peculiar, complicated structure, and that in the greater number of Acantharia it becomes cleft very early, and that this cleavage is effected by a peculiar kind of gemmation, first detected and very accurately described by R. Hertwig (compare his Organismus d. Radiol., 1879, pp. 10-24). However, in the young Acantharia the nucleus is constantly simple, and in a certain number of species its cleavage takes place late (as in the greater number of Spumellaria).

The Endoplasm, or the intracapsular sarcode, exhibits in the greater number of Acantharia a more or less distinct radial arrangement; but this is often concealed by the different enclosed products of the endoplasm—oil-globules, vacuoles, red or different coloured pigment-granules, crystals, &c. Often it encloses a variable number of "yellow cells" (becoming green by mineral acids) to be considered as symbiotic xanthellæ.

The Calymma or the jelly-veil, including the central capsule, in the Acantharia is more or less voluminous, and commonly envelops the skeleton perfectly. In its surface is sometimes developed a peculiar network of "supporting fibres." A very peculiar product are the remarkable "Myophrisca" of the Acanthometra, which are wanting in the Acanthophracta; they were first detected by Johannes Müller, and figured as "Cilien-Kränze," afterwards explained by Hertwig as "contractile Fäden," similar to muscular fibrillæ (compare below).

The Matrix, placed between the calymma and central capsule, in the majority of the Acantharia is a rather thin layer of granular exoplasm.

The Pseudopodia arising from it are not so numerous as in the Spumellaria, and not so equally disposed over the whole surface. Also their tendency to ramify, anastomose, and form networks seems to be much less developed. Commonly they are simple or little ramified. In many cases (and perhaps everywhere) there may be distinguished two different kinds of pseudopodia:—(1) Axopodia, or permanent pseudopodia (with axial filaments?), piercing the wall of the central capsule, and arising from the central mass of endoplasm; and (2) Collopodia, or variable pseudopodia (without axial filaments), arising outside the capsule from the matrix of extracapsular sarcode or from the exoplasm on the surface of the calymma. These and other differentiations seem to indicate that the pseudopodia in the Acantharia are more highly developed than in the Spumellaria, and justify the denomination of the former as "Actipylea."

Synopsis of the Orders and Suborders of Acantharia.

II. ACANTHOMETRA. Skeleton composed only of acanthinic radial spines not forming a complete lattice-shell. Radial spines in variable and indefinite number, disposed irregularly, 1. Actinelida.

Radial spines constantly twenty, disposed regularly after the Müllerian law of Icosacantha, 2. Acanthonida.

II. ACANTHOPHRACTA. Skeleton composed of twenty acanthinic radial spines (disposed after the Müllerian law) and of a spherical or variously shaped complete lattice-shell. Radial spines all twenty of equal size; shell and central capsule spherical, 3. Sphærophracta.

Radial spines of different sizes; shell and central capsule ellipsoidal, discoidal, or heteromorphous, 4. Prunophracta.

I. ACANTHOMETRA. Skeleton composed only of acanthinic radial spines not forming a complete lattice-shell.

Radial spines in variable and indefinite number, disposed irregularly,

1. Actinelida.

Radial spines constantly twenty, disposed regularly after the Müllerian law of Icosacantha,

2. Acanthonida.

II. ACANTHOPHRACTA. Skeleton composed of twenty acanthinic radial spines (disposed after the Müllerian law) and of a spherical or variously shaped complete lattice-shell.

Radial spines all twenty of equal size; shell and central capsule spherical,

3. Sphærophracta.

Radial spines of different sizes; shell and central capsule ellipsoidal, discoidal, or heteromorphous,

4. Prunophracta.

Order III. ACANTHOMETRA, Johannes Müller, 1855.

Definition.—Acantharia without complete latticed shell.

The order Acanthometra, the third order of Radiolaria, comprises all those Acantharia in which the acanthinic skeleton is only composed of radial spines arising from one common central point, but never forms a complete latticed shell. By the absence of such a latticed or fenestrated shell the Acanthometra differ principally from the nearly allied Acanthophracta, the second order of Acantharia, which constantly possess such a complete shell.

Johannes Müller, who first detected and described the Acanthometra (in 1855, loc. cit.), defined them as follows:—"Radiolaria without shell, with siliceous radial spines" (1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 46). He described and figured eighteen species of them, disposed in four genera (Acanthometra with fifteen species, and Zygacantha, Lithophyllium, Lithoptera, each with a single species). Among those eighteen species, however, were two "Acanthometræ cataphractæ," appertaining to the following order, the Acanthophracta.

In my Monograph (1862, p. 371) all true Acanthometra were united into a single family, Acanthometrida, with the following definition:—"Skeleton composed of a number of radial spines, piercing the central capsule and united in its centre, without latticed shell." In the majority of them I observed that the skeleton did not consist of silex, but of a very peculiar organic substance, which I called "acanthin." At that time I divided the family Acanthometrida into four subfamilies:—(1) Acanthostaurida, (2) Astrolithida, (3) Litholophida, (4) Acanthochiasmida. The two former now represent the suborder Acanthonida, the two latter the suborder Actinelida. The number of genera which I distinguished in my Monograph amounted to nine, the number of species to fifty. By the rich collections of the Challenger this number is so much increased that we can here describe twenty-seven genera and one hundred and sixty species.

Richard Hertwig in his work on the Organismus der Radiolarien (1879, pp. 6-25) adopted my family Acanthometrida, and gave a very accurate description of its anatomical structure. He confirmed my observations that the radial spines of this family are never hollow, but solid, and that their chemical substance is not silex, but the organic matter "acanthin." He found that the simple nucleus of the Acanthometrida is commonly very early cleft, and that the peculiar brushes of filaments on the calymma, described by Johannes Müller and by me as "Gallert-cilien," are peculiar "contractile filaments," comparable to the "muscle-fibrillæ" of some Infusoria, or the "Myophan-filaments" (Myophrisca).

The order Acanthometra is here divided into two different suborders of very unequal extent and value, the Actinelida and Acanthonida. The first may be regarded as the common ancestral stock, not only of the second, but of all Acantharia. In the small group of Actinelida the number of radial spines is variable and commonly indefinite, often very large (more than a hundred); they are therefore Adelacantha. The second suborder, the Acanthonida, comprise by far the greatest part of the order, and possess constantly twenty radial spines, regularly disposed after the Müllerian law; they are therefore (like all Acanthophracta) Icosacantha (compare above, p. 717).

The Actinelida possess constantly simple radial spines, without any apophyses; their form is commonly very simple and primitive. This suborder comprises three small but very different families, the Astrolophida, Litholophida, and Chiastolida. The first family, the Astrolophida, is the original ancestral group. A large and variable, commonly indefinite number of radial spines is here united in the centre of the spherical central capsule and radiating within a spherical space. In the second family, the Litholophida, a small and variable number of radial spines (between ten and twenty) is united in the apex of a conical central capsule and radiating within the quadrant or octant of a spherical space. In the third family, the Chiastolida, a variable number of radial spines is grown together by pairs, in such a manner that every two opposite spines (placed originally in one axis of the spherical central capsule) forms a single "diametral spine"; all these diametral spines are not united in the centre of the central capsule but only crossed loosely near the centre.

The Acanthonida, the second suborder of Acanthometra, embraces by far the greatest number in this order, viz., all those forms in which twenty radial spines are regularly disposed after the Müllerian law—Icosacantha (compare above, p. 717). The radial spines of this suborder are either simple or provided with transverse processes (either two opposite or four crossed apophyses). They are commonly united in the middle of the central capsule by their opposed basal ends, forming small pyramids; the meeting triangular faces of the neighbouring pyramids being propped one upon another. Above these small basal pyramids often arises a basal leaf-cross formed by four broad triangular leaves or wings with straight edges; the meeting thin edges of the neighbouring spines serve for strengthening the basal junction and form hollow pyramidal spaces or compartments, filled with the contents of the central capsule (compare p. 721). The suborder Acanthonida comprises three different families, the Astrolonchida, Quadrilonchida, and Amphilonchida. The first family, the Astrolonchida, comprises by far the greater number of the Acanthonida; those genera in which all twenty spines are perfectly equal or nearly equal in size and form. In the second family, the Quadrilonchida, the four equatorial spines are much larger (and often also of another form) than the sixteen other spines (often also the eight tropical larger than the eight polar spines). The third family, the Amphilonchida, is distinguished by the preponderating development of only two opposite equatorial spines, which are much larger (and often also of another form) than the eighteen other spines.

Synopsis of the Suborders and Families of Acanthometra.

Suborder I. ACTINELIDA. Number of the radial spines variable, either more or less than twenty, commonly disposed irregularly and not according to the Müllerian law. Radial spines very numerous (thirty to a hundred or more), radiating from a common centre within a spherical space, 1. Astrolophida.

Radial spines between ten and twenty, radiating from one common point within a sphere-quadrant, 2. Litholophida.

Radial spines of variable number; every two opposite spines grown together in the centre; therefore numerous diametral spines are crossed freely in the centre, 3. Chiastolida.

Suborder II. ACANTHONIDA. Number of the radial spines constantly twenty, disposed regularly according to the Müllerian law. All twenty radial spines nearly equal, and of the same size and form, 4. Astrolonchida.

Four equatorial spines much larger than (and often also of different form from) the sixteen other spines, 5. Quadrilonchida.

Two opposite equatorial spines (or principal spines) much larger than (and often also of different form from) the eighteen other spines, 6. Amphilonchida.

Suborder I. ACTINELIDA. Number of the radial spines variable, either more or less than twenty, commonly disposed irregularly and not according to the Müllerian law.

Radial spines very numerous (thirty to a hundred or more), radiating from a common centre within a spherical space,

1. Astrolophida.

Radial spines between ten and twenty, radiating from one common point within a sphere-quadrant,

2. Litholophida.

Radial spines of variable number; every two opposite spines grown together in the centre; therefore numerous diametral spines are crossed freely in the centre,

3. Chiastolida.

Suborder II. ACANTHONIDA. Number of the radial spines constantly twenty, disposed regularly according to the Müllerian law.

All twenty radial spines nearly equal, and of the same size and form,

4. Astrolonchida.

Four equatorial spines much larger than (and often also of different form from) the sixteen other spines,

5. Quadrilonchida.

Two opposite equatorial spines (or principal spines) much larger than (and often also of different form from) the eighteen other spines,

6. Amphilonchida.

Suborder I. ACTINELIDA, Haeckel, 1882.

Definition.—Acanthometra with a variable number of radial spines, which are commonly irregularly disposed, not according to the Icosacantha.

Family XXXIII. Astrolophida, Haeckel.

Astrolophida, Haeckel, 1881, Prodromus, p. 469.

Definition.—Acantharia with a variable number of simple radial spines, radiating within a spherical space from one common central point which is the centre of the spherical central capsule. No lattice shell.

The family Astrolophida comprises the simplest and the most primitive forms among all Acantharia, and may therefore be regarded as the common ancestral stock of this whole legion or subclass of Radiolaria. The acanthinic skeleton is composed of a variable number of quite simple radial spines, which are united in the centre of the spherical central capsule and radiate, piercing its walls and the surrounding jelly-veil, within a spherical space.

The first observed form of this family is the ancestral genus Actinelius, two different species of which I detected in 1864 in the northern Mediterranean, at Villafranca, near Nice (compare Zeitschr. f. wiss. Zool., 1865, Bd. xv. p. 364, Taf. xxvi. fig. 4). Three other species of the same genus were afterwards found by me in the Challenger collections. Whilst in this Actinelius all radial spines are of the same size, a new nearly allied genus, Astrolophus (with two species), differs from it by the different size of the radial spines, a small number of very large spines being intermingled with a very large number of small spines. In these two genera, Actinelius and Astrolophus (the true "Astrolophida" sensu strictiori), the number of the radial spines is quite indeterminable and their arrangement quite irregular and variable.

A third remarkable genus, Actinastrum, differs from these two genera in the definite number and regular order of thirty-two radial spines, and may therefore perhaps better represent a peculiar family, Actinastrida. In this genus (of which two species were observed) the thirty-two radial spines are disposed in such a regular manner that they lie in four meridian planes, and that their distal ends fall into five parallel zones. These five zones and these four planes are the same as we find in all Icosacantha (compare above, p. 717). Also the constant twenty spines of these latter are present in Actinastrum; but their number is here enlarged by twelve other spines missing in the Icosacantha; four of these are secondary or interradial equatorial spines, lying opposite in pairs between the four primary or perradial equatorial spines; and eight are perradial tropical spines, lying between the eight interradial tropical spines. Therefore the distal ends of the thirty-two radial spines are disposed regularly in five parallel zones, and while two zones (the two polar) contain only the points of every four spines, three zones (the single equatorial and the two tropical) contain the points of every eight spines. The four meridian planes are in Actinastrum the same as in the Icosacantha, crossed in the spineless axis at angles of 45°. But in the Icosacantha each of the two perradial meridian planes contains six radial spines (two equatorial and four polar), each of the two interradial meridian planes only four tropical spines. Whereas in Actinastrum each of the two primary or perradial meridian planes contains ten spines (two equatorial, four tropical, and four polar), each of the two secondary or interradial meridian planes six spines (two equatorial and four tropical). We find therefore altogether thirty-two radial spines in three orders; eight equatorial, sixteen tropical, and eight polar spines.

Only one other genus of Radiolaria exhibits the same characteristic disposition of thirty-two radial spines as Actinastrum, and this is Chiastolus; but here the two opposite spines of each pair are grown together and form one diametral spine; and the sixteen diametral spines are crossed in the centre of the capsule. In Actinastrum, as in Astrolophus and Actinelius, the central ends or bases of all the spines are pyramidal, and the triangular faces of the neighbouring spines rest one upon another (as in the greater number of Acanthonida). The form of the radial spines in all Astrolophida is quite simple, without lateral processes or apophyses; chiefly cylindrical, more rarely compressed, two-edged or quadrangular.

The central capsule in all Astrolophida is spherical, and in the younger specimens contains a single large concentric and lobed nucleus, but in the older specimens a large number of small nuclei. The surrounding jelly-veil or calymma seems commonly to envelop the spines perfectly. The piercing pseudopodia radiate everywhere between the spines, and are very numerous and thin. The circulating granules in them are sometimes red (Actinelius purpureus).

Synopsis of the Genera of Astrolophida.

Radial spines of indefinite number and of irregular disposition. Spines of equal size, 317. Actinelius.

Spines of unequal size, 318. Astrolophus.

Radial spines thirty-two, disposed regularly in five parallel zones, 319. Actinastrum.

Radial spines of indefinite number and of irregular disposition.

Spines of equal size,

317. Actinelius.

Spines of unequal size,

318. Astrolophus.

Radial spines thirty-two, disposed regularly in five parallel zones,

319. Actinastrum.

Genus 317. Actinelius,^[357] Haeckel, 1865, Zeitschr. f. wiss. Zool., Bd. xv. p. 364.

Definition.—Astrolophida with a variable and undetermined number of simple radial spines, all of equal size, united in the centre of the spherical central capsule.

The genus Actinelius comprises the most simple and primitive forms among all Acantharia, and may be regarded as the common ancestral stock of this whole legion. The spherical central capsule is pierced by numerous simple radial spines of equal size, the pyramidal bases of which are supported one upon another with their triangular faces in the centre of the capsule. The number and position of the spines are quite indefinite and variable. We may derive Actinelius either from Actissa (Colloidea) by development of acanthinic radial spines, or directly from Actinosphærium (Heliozoa) by formation of a central capsule.

Subgenus 1. Actinelarium, Haeckel.

Definition.—Radial spines cylindrical, conical, or spindle-shaped, their transverse section circular.

1. Actinelius primordialis, n. sp. (Pl. 129, fig. 1).

Spines sixty to eighty or more, cylindrical, at the distal end thickened, spindle-shaped. Apex simple. Base a small slender pyramid. Central capsule yellow. Granules of the sarcode colourless.

Dimensions.—Length of the spines 0.3 to 0.4, breadth in the distal part 0.02, in the basal part 0.008.

Habitat.—Central Pacific, Stations 265 to 274, surface.

2. Actinelius purpureus, Haeckel.

Actinelius purpureus, Haeckel, 1865, Zeitschr. f. wiss. Zool., Bd. xv. p. 364, Taf. xxvi. fig. 4.

Spines thirty to forty or more, cylindrical, very thin, a little thinner towards both ends. Apex simple. Base a small sulcate pyramid. Central capsule opaque, purple. Granules of the sarcode also purple.

Dimensions.—Length of the spines 0.2 to 0.3, breadth 0.002.

Habitat.—Mediterranean (Nice), Haeckel.

Subgenus 2. Actinelidium, Haeckel.

Definition.—Radial spines compressed, two-edged; their transverse section elliptical or lanceolate.

3. Actinelius protogenes, n. sp.

Spines fifty to sixty, compressed, two-edged, gradually broadened towards the truncated distal end. Basal or proximal end thin, pyramidal. The spines of this species are similar to those of Actinastrum pentazonium (p. 733) and of Chiastolus amphicopium (Pl. 129, fig. 3), but much more numerous, smaller, and not regularly disposed. These latter two Actinelida must be separated on account of the regular disposition of the thirty-two spines.

Dimensions.—Length of the spines 0.2, basal breadth 0.008, distal breadth 0.02.

Habitat.—South Pacific, Station 165, surface.

Subgenus 3. Actinelonium, Haeckel.

Definition.—Radial spines quadrangular, prismatic, or pyramidal, their transverse section square.

4. Actinelius pallidus, Haeckel.

Actinelius pallidus, Haeckel, 1865, Zeitschr. f. wiss. Zool., Bd. xv. p. 364.

Spines eighty to one hundred and twenty or more, quadrangular, prismatic, of equal breadth throughout their whole length. Apex simple, truncate or pyramidal. Base a four-sided slender pyramid. Central capsule pale yellowish. Granules of the sarcode colourless.

Dimensions.—Length of the spines 0.2 to 0.3, breadth 0.005.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific.

5. Actinelius polyacanthus, n. sp.

Spines two hundred to three hundred or more, quadrangular, pyramidal, gradually thinned towards the simple apex. Base a small three-sided pyramid. Central capsule opaque.

Dimensions.—Length of the spines 0.12 to 0.18, basal breadth 0.012.

Habitat.—South Pacific, Station 291, surface.

Genus 318. Astrolophus,^[358] Haeckel, 1881, Prodromus, p. 469.

Definition.—Astrolophida with a variable and undetermined number of simple radial spines of different sizes (large and small spines intermingled), which are united in the centre of the spherical central capsule.

The genus Astrolophus differs from the nearly allied ancestral genus Actinelius only in the unequal size of the numerous radial spines. In both observed species very numerous small spines are intermingled with a small number of large spines, and between them numerous spines of medium size. The small spines fill up the hollow spaces between the basal parts of the large spines.

1. Astrolophus stellaris, n. sp.

Radial spines from one hundred to two hundred, of very different sizes, but of similar form; about sixteen to twenty very large spines, forty to fifty of medium size, and one hundred to one hundred and twenty much smaller. All spines cylindrical in the greater part of their length, with simple apex, gradually thickened towards the central part, conical, without edges. The base itself is a slender pyramid with four to eight edges.

Dimensions.—Length of the largest spines 0.3 to 0.4, of the majority 0.1 to 0.2, of the smallest 0.05 to 0.1.

Habitat.—South Pacific, Station 288, surface.

2. Astrolophus solaris, n. sp. (Pl. 132, figs. 12a, 12b).

Radial spines from two hundred to three hundred, of very different sizes, but of similar form; about twenty to thirty very large spines, sixty to eighty of medium size, and one hundred and twenty to one hundred and fifty much smaller. All spines cylindrical in the greater part of their length, with simple apex, gradually thickened and four-edged towards the central base. The base itself is a slender pyramid with four to eight edges; partly the faces, partly the edges of these basal pyramids rest one upon another, the points of the larger spines meeting in the centre.

Dimensions.—Length of the largest spines 0.4 to 0.5, of the majority 0.2 to 0.3, of the smallest 0.1 to 0.16; basal thickness of the largest spines 0.015.

Habitat.—South-east Pacific (off Juan Fernandez), Station 296, surface.

Genus 319. Actinastrum,^[359] n. gen.

Definition.—Astrolophida with thirty-two simple radial spines, regularly disposed within four meridian planes in such an order that their distal ends fall into five parallel zones. Central ends of the thirty-two spines supported one upon another in the centre of the spherical central capsule.

The genus Actinastrum differs from the two preceding genera in the definite number and order of the thirty-two radial spines, which are disposed in a very remarkable manner. Twenty radial spines are disposed after the Müllerian law of Icosacantha (compare above, p. 717). The remaining twelve spines are four equatorial spines lying in the two secondary meridian planes, and eight tropical spines lying in the two primary meridian planes. We have therefore together eight equatorial, sixteen tropical, and eight polar spines (compare above, p. 729).

1. Actinastrum legitimum, n. sp.

All thirty-two radial spines of equal size and similar form, cylindrical, conical at the distal end, at the central base pyramidal. Central capsule pellucid, colourless.

Dimensions.—Length of the radial spines 0.3, breadth 0.004.

Habitat.—South Pacific, Station 288, surface.

2. Actinastrum pentazonium, n. sp.

All thirty-two radial spines of equal size and similar form, compressed, two-edged, gradually becoming broader and thinner from the pyramidal central base towards the truncated distal end. Central capsule dark, opaque. (Compare the similar Chiastolus amphicopium, Pl. 129, fig. 3.)

Dimensions.—Length of the radial spines 0.2, breadth at the base 0.005, at the distal end 0.02.

Habitat.—South Pacific (west coast of Patagonia), Station 302, surface.

Family XXXIV. Litholophida, Haeckel.

Litholophida, Haeckel, 1862, Monogr. d. Radiol., p. 401.

Definition.—Acantharia with a variable number of simple radial spines radiating within a conical space (or within the quadrant of a sphere) from one common central point, which is the apex of the conical central capsule. No lattice-shell.

The family Litholophida, represented only by a single genus, Litholophus, differs from all other Acantharia in the remarkable fact that the common point, from which the radial spines arise, is not the geometrical central point of the whole body, but is quite excentric in position, the apex of the conical or pyramidal central capsule. Therefore the spines form together a kind of brush or broom.

When I founded the family Litholophida in my Monograph (1862, p. 401) I knew only a single species, Litholophus rhipidium, observed very frequently in Messina. Another species, Litholophus ligurinus, was afterwards (1864) found by me at Nice. Six other species were detected in the preparations of the Challenger, some of them very frequent. All these eight species of Litholophus are very nearly allied, and exhibit only slight differences in the form and number of the radial spines; their mode of excentric connection and the structure of the peculiar soft body is everywhere the same.

The radial spines in all observed Litholophida possess the form of the genus Acanthonia, i.e., they are quite simple, four-sided prismatic or quadrangular, with square transverse section; their four edges are sometimes smooth, at other times elegantly denticulate, commonly more or less prominent or wing-shaped. In the greater number of species they are very long and of nearly equal breadth, prismatic; in some species they are more pyramidal, thinned towards the distal end; the latter is commonly truncated or broken off, sometimes pyramidal. The central end is everywhere thinned, more or less pyramidal, and the neighbouring spines are propped one upon another by the triangular faces of their small basal pyramids. A slight pressure is sufficient to destroy their connection.

The number and disposition of the radial spines seem to be variable and irregular, but require further researches. In four of the observed eight species I found constantly ten spines, in two other species from ten to twenty (commonly twelve or sixteen), and in two species twenty or more. A certain order or disposition of the spines within the conical space in which they radiate could nowhere be ascertained.

When I first observed Litholophus, I supposed that it might only be a mutilated or altered form of an Acanthonia. Afterwards, observing many specimens with ten spines, I was led to the suggestion that they were produced by self-division of an Acanthonia, and that the number of the spines in each half of the body might be afterwards doubled. But this suggestion seems to be refuted by the fact that in no other genus of the numerous Acantharia is self-division observed, and that many hundreds of Litholophus which I observed exhibit quite constantly only a single form of radial spines, that of Acanthonia—simple quadrangular spines without any apophyses.

Genus 320. Litholophus,^[360] Haeckel, 1862, Monogr. d. Radiol., p. 401.

Definition.—Litholophida with a variable number of quadrangular diverging radial spines, united with pyramidal bases in the apex of the conical central capsule.

The genus Litholophus, the only one of this family, exhibits the peculiarities just described, but might more nearly be defined as a typical "genus" by the quadrangular form of the radial spines, identical with those of Acanthonia.

The central capsule of Litholophus is constantly conical or pyramidal, commonly opaque, of a dark brownish or reddish colour; it contains many small nuclei. It envelops the basal half of all radial spines in such a manner that their basal parts are united in its apex, and their distal parts pierce the rounded base of the conical capsule (Pl. 129, fig. 2).

The calymma or the jelly envelope of the central capsule is only developed at its base, where the spines radiate; at the conical mantle of the capsule it is very thin. The spines seem to be perfectly enclosed in the calymma and connected with it by the same contractile retinacula or "myophrisca" which we observe in the Acanthonida. The pseudopodia arise only from the rounded base of the conical capsule, and radiate between the spines, piercing the calymma, diverging within the conical space occupied by the fascicle of spines.

Subgenus 1. Litholopharium, Haeckel.

Definition.—Ten radial spines.

1. Litholophus decimalis, n. sp.

Ten radial spines, four-sided prismatic, with prominent smooth edges, of equal breadth throughout their whole length.

Dimensions.—Length of the spines 0.2 to 0.3, breadth 0.006.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific, surface.

2. Litholophus pyramidalis, n. sp.

Ten radial spines, four-sided pyramidal, with prominent smooth edges, gradually thickened from the small pyramidal base towards the truncated distal end.

Dimensions.—Length of the spines 0.3 to 0.4, breadth in the basal part 0.002, in the middle part 0.006, in the distal part 0.012 to 0.02.

Habitat.—Central Pacific, Station 266, surface.

3. Litholophus decapristis, n. sp. (Pl. 129, fig. 2).

Ten radial spines, four-sided prismatic, with prominent, elegantly denticulated edges, of equal breadth in their whole length.

Dimensions.—Length of the spines 0.2 to 0.4, breadth 0.008.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface.

4. Litholophus decastylus, n. sp.

Ten radial spines, four-winged pyramidal, with broad and thin, elegantly denticulated edges, gradually thickened from the small pyramidal base towards the truncated distal end.

Dimensions.—Length of the spines 0.2 to 0.3, breadth in the basal part 0.002, in the middle part 0.005, in the distal part 0.015.

Habitat.—South Atlantic, Station 332, surface.

Subgenus 2. Litholophidium, Haeckel.

Definition.—Number of the radial spines variable, between ten and twenty, commonly twelve to sixteen.

5. Litholophus ligurinus, Haeckel.

Litholophus ligurinus, Haeckel, 1865, Zeitschr. f. wiss. Zool., Bd. xv. p. 366.

Spines of variable number, from eleven to twenty, commonly twelve to sixteen, four-sided prismatic, with smooth thin edges, of equal breadth in their whole length or a little thinner towards the proximal end.

Dimensions.—Length of the spines 0.3, breadth 0.005.

Habitat.—Mediterranean (Nice); Central Pacific, Station 274, surface.

6. Litholophus rhipidium, Haeckel.

Litholophus rhipidium, Haeckel, 1862, Monogr. d. Radiol., p. 402, Taf. xix. fig. 6.

Spines of variable number, from eleven to twenty, commonly twelve to sixteen, four-sided prismatic, with distantly denticulated edges, of equal breadth in their whole length or a little thinner towards both ends.

Dimensions.—Length of the spines 0.3, breadth 0.006.

Habitat.—Mediterranean (Messina); North Atlantic, Station 352, surface.

Subgenus 3. Litholophonium, Haeckel.

Definition.—Number of the radial spines twenty (or more?).

7. Litholophus fasciculus, n. sp.

Spines constantly (?) twenty, four-sided prismatic, with smooth prominent edges, nearly of equal breadth throughout their whole length.

Dimensions.—Length of the spines 0.3 to 0.5, breadth 0.008.

Habitat.—South Pacific, Station 291, surface.

8. Litholophus penicillus, n. sp.

Spines constantly twenty (or more?), four-sided prismatic, with distantly denticulated edges, gradually thickened from the small pyramidal base to the middle part, of equal breadth in the distal half.

Dimensions.—Length of the spines 0.3 to 0.4, breadth 0.005 to 0.007.

Habitat.—North Pacific, Station 252, surface.

Family XXXV. Chiastolida, Haeckel.

Acanthochiasmida, Haeckel, 1862, Monogr. d. Radiol., p. 402.

Definition.—Acantharia with a variable number of simple radial spines, which are grown together in pairs (two opposite spines of each pair representing together a single diametral spine). Diametral spines crossed loosely in the centre of the spherical or irregular roundish central capsule. No lattice-shell.

The family Chiastolida (or Acanthochiasmida) differs from all other Acantharia in the peculiar mode of the radial spines; these grown together in pairs in the centre of the body, so that every two spines opposite in one axis of the body form together one single diametral spine. All diametral spines are loosely crossed in the middle of the central capsule, or connected by a peculiar screw-like winding, but not united firmly.

I established the family Acanthochiasmida in my Monograph (1862, p. 402) upon the single genus Acanthochiasma (with three species), in which only ten diametral spines are constantly found; I derived these from the twenty radial spines of the common Acanthometra, supposing that every two opposite spines of the latter (lying in one axis) were grown together in the centre, whilst the intimate connection of the twenty radial spines in the common centre was dissolved. This opinion was afterwards confirmed by Richard Hertwig, who observed Acanthochiasma intact in the living state. Although two species of this genus are cosmopolitan and very common, the number of species is very small; I could add to those three older known forms only a single new species.

Another genus of this family, Chiastolus, was observed by me only in a single specimen but it is extremely interesting. It has sixteen diametral spines, disposed quite regularly after the same law of the thirty-two spines of Actinastrum which we described above (compare above, p. 729). Therefore we cannot doubt that the former is derived from the latter in the same way, every two opposite radial spines (of one axis) being grown together to form a single diametral spine. As we place Acanthometron (with twenty spines) and Actinastrum (with thirty-two spines) in two different families, it would perhaps be more convenient to separate also Acanthochiasma and Chiastolus as representatives of two different families—Acanthochiasmida (with ten diametral spines) and Chiastolida (with sixteen diametral spines).

As we derive Acanthochiasma from Acanthometron by concrescence in pairs of the twenty radial spines, the Müllerian law of Icosacantha must be employed also to the ten diametral spines of the former, therefore two of them are equatorial, four tropical, and four polar spines. In the same way we may employ the new law of disposition found in the thirty-two radial spines of Actinastrum equally to the sixteen diametral spines of Chiastolus, which we derive from the former, four of them are equatorial, eight tropical, and four polar spines (compare above, p. 732).

The Central Capsule of the Chiastolida is spherical, and exhibits in general the same shape as in the Acanthonida, and specially in the Astrolonchida. Of course every diametral spine pierces the capsule twice, at two points diametrically opposed. In some species of Acanthochiasma the central capsule is formed very late, so that it seems often to be absent. A very accurate description of the capsule and its nucleus, as well as of the calymma and the pseudopodia, is given by Richard Hertwig in his Organismus der Radiolarien (1879, pp. 10-18). The pseudopodia are very numerous, and sometimes bear reddish granules.

Synopsis of the Genera of Chiastolida.

I. Subfamily Chiastolidina. Sixteen diametral spines, derived by concrescence of thirty-two radial spines, 321. Chiastolus.

II. Subfamily Acanthochiasmida. Ten diametral spines, derived by concrescence of twenty radial spines, 322. Acanthochiasma.

Genus 321. Chiastolus,^[361] n. gen.

Definition.—Chiastolida with sixteen diametral spines, derived from thirty-two radial spines opposite and grown together in pairs.

The genus Chiastolus, hitherto known only by a single observed specimen, comprises the Chiastolida with sixteen diametral spines, which are loosely crossed in the centre of the body. Four of these spines are equatorial, four polar, and eight tropical. We derive Chiastolus from Actinastrum by concrescence of every two opposite spines in one axis of the body (compare above, pp. 729, 732).

1. Chiastolus amphicopium, n. sp. (Pl. 129, figs. 3, 3a, 3b).

Spines in the central part cylindrical, spirally convoluted in a very peculiar manner, broadened towards both ends, strongly compressed, two-edged; the broadest parts are the two truncated distal ends, five to seven times as broad as the thinnest central part, which is placed between two spindle-shaped intumescences. These cochleary central parts of the sixteen spines seem to be resting one upon another. Each spine (composed of two opposite equal radial spines) has nearly the form of a double oar. The single observed specimen (preserved in glycerine) exhibited a most regular disposition of the thirty-two spines (grown together in pairs in the centre). The diameter of the dark non-transparent spherical central capsule equalled one-fifth to one-fourth of the total length of the double spines. When the soft parts of the body were destroyed by sulphuric acid, the sixteen single spines were suddenly dispersed.

Dimensions.—Length of the sixteen double spines 0.5, distal breadth (of the truncated ends) 0.05, central breadth 0.01; diameter of the central capsule 0.12.

Habitat.—South-east Pacific (near Juan Fernandez), Station 297, surface.

Genus 322. Acanthochiasma,^[362] Krohn, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 810.

Definition.—Chiastolida with ten diametral spines, derived from twenty radial spines opposite and grown together in pairs.

The genus Acanthochiasma with a small number of common species, comprises the Chiastolida with ten diametral spines, which are loosely crossed in the centre of the body. I could distinguish only four species, two of which are cosmopolitan and very widely distributed. In all four species the diametral spines are quite simple, cylindrical; only in one species distinguished by a spiral winding or torsion in the middle part, where they are crossed one to another. We derive Acanthochiasma from Acanthometron by concrescence of every two spines opposite in one axis of the body.

1. Acanthochiasma krohnii, Haeckel.

Acanthochiasma krohnii, Haeckel, 1862, Monogr. d. Radiol., p.403, Taf. xix. fig. 7.

Acanthochiasma krohnii, R. Hertwig, 1879, Organismus d. Radiol., Taf. ii. fig. 6.

Spines needle-shaped, cylindrical, very thin and long, of equal breadth in their whole length, distinguished by a high degree of elasticity. Central capsule colourless or yellowish-white, transparent. Granules of the sarcode colourless.

Dimensions.—Length of the spines 0.5 to 1.0, breadth 0.001 to 0.002.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific, very common.

2. Acanthochiasma rubescens, Krohn.

Acanthochiasma rubescens, Haeckel, 1862, Monogr. d. Radiol., p. 403.

Spines cylindrical, of equal breadth in their whole length, not very elastic, pointed at the two ends. Central capsule intransparent, reddish, with violin-shaped concretions. Granules of the sarcode red coloured.

Dimensions.—Length of the spines 0.2 to 0.6, breadth 0.004 to 0.006.

Habitat.—North Atlantic, Madeira, Krohn; Lanzerote, Haeckel.

3. Acanthochiasma fusiforme, Haeckel.

Acanthochiasma fusiforme, Haeckel, 1862, Monogr. d. Radiol., p. 404, Taf. xix. fig. 8.

Spines spindle-shaped, from the thicker central part thinned towards the two thin conical ends, perfectly straight and smooth, rigid, inelastic. Central capsule non-transparent, brown.

Dimensions.—Length of the spines 0.3 to 0.5, breadth in the central part 0.006 to 0.009.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific.

4. Acanthochiasma spirale, n. sp.

Spines spindle-shaped, tapering from the thicker central part towards the two thin conical ends, rigid, inelastic; their central part is spirally convoluted in a very peculiar cochlea-like manner, as in Chiastolus amphicopium (Pl. 129, figs. 3a, 3b). The ten spines are propped one upon another by the central screw. Central capsule dark, opaque.

Dimensions.—Length of the spines 0.2 to 0.3, breadth of the central spiral part 0.01.

Habitat.—Central Pacific, Station 266, surface.

Suborder II. ACANTHONIDA, Haeckel (Pls. 130-132).

Acanthonida, Haeckel, 1881, Prodromus, p. 465.

Definition.—Acanthometra with twenty radial spines, disposed according to the Müllerian or Icosacanthan law in five zones each of four spines.

Family XXXVI. Astrolonchida, Haeckel (Pl. 130).

Astrolonchida, Haeckel, 1881, Prodromus, p. 465.

Definition.—Acanthometra with twenty radial spines of nearly equal size and similar form, disposed according to the law of the Icosacantha. No lattice-shell.

The family Astrolonchida, the first and oldest of the Acanthonida, is no doubt the ancestral stock not only of this suborder but also all Acanthophracta, i.e., of all Icosacantha or all Acantharia in which twenty radial spines are regularly disposed according to the Müllerian law, forming five zones each of four alternating spines (compare above, p. 717). The Astrolonchida differ from the Acanthophracta in the absence of a complete lattice-shell, from the other two families of Acanthonida (the Quadrilonchida and Amphilonchida) in the equal size and similar form of all the spines. Probably this equality is nowhere quite perfect, since in all Icosacantha the central bases of the twenty spines exhibit originally certain slight differences of form and junction, effected by the regular disposition itself. But setting aside this slight difference, only recognisable by means of a very accurate investigation of the central junction (and in thinner spines often not at all recognisable), the twenty spines of the Astrolonchida appear perfectly equal. Therefore the four equatorial spines are not distinguished from the sixteen other spines, as is constantly the case in the two following families.

The number of genera (eleven) and of species (seventy-six) in the Astrolonchida is far larger than in the five other families of Acanthometra, and requires a distinction into three different subfamilies. (A) In the Zygacanthida the form of the radial spines is quite simple, without apophyses or transverse processes; (B) in the Phractacanthida each spine bears two opposite apophyses (rarely two longitudinal rows of these opposite apophyses); (C) in the Stauracanthida each spine bears a cross of four apophyses, opposite in pairs (rarely four longitudinal crossed rows of apophyses, opposite in pairs). The Phractacanthida and Stauracanthida appear as two divergent branches of the pedigree, derived independently from the common ancestral stock of Zygacanthida.

In the Zygacanthida, constantly devoid of apophyses, we can distinguish only three genera, characterised by the different fundamental form of the radial spines; these are:—(1) Acanthometron, with cylindrical or conical spines (without edges); (2) Zygacantha, with compressed and two-edged spines; (3) Acanthonia, with four-edged, prismatic or pyramidal spines. The transverse section of the spines is in the first case circular, in the second elliptical or lanceolate, in the third square. All the different forms of spines, which we find in the numerous Acantharia, may be reduced to these three forms, and among these the second and third are derived from the first.

The development of apophyses or of lateral transverse processes (wanting in the Zygacanthida) is of the greatest value for the further differentiation of the Acantharia. For from the Phractacanthida (with two opposite apophyses on each spine) we must derive the Phrastaspida, the common ancestral stock of the Diporaspida (and therefore also the Belonaspida, Hexalaspida, Diploconida, and Phractopeltida). On the other hand the Stauracanthida (with four crossed apophyses on each spine) have produced the Stauraspida, or the ancestral group of the Tessaraspida and Sphærocapsida. From all these Acanthophracta, possessing a complete lattice-shell, the Astrolonchida differ in the absence of such a complete shell. Also in the few cases in which the apophyses become latticed (Doracantha among the Phractacanthida, and Phatnacantha among the Stauracanthida), the lattice-plates of the neighbouring spines never meet with their edges, as is the case in all Acanthophracta. But in a phylogenetic as well as in an ontogenetic sense the former are the ancestral stock of the latter.

The Central Capsule in the Astrolonchida is commonly spherical, sometimes with twenty roundish elevations or conical papillæ, extending radially to the basal half of the radial spines. The calymma is voluminous, and forms around the radial spines conical or cylindrical "jelly-sheaths," which are connected with the spines by coronas of Myophrisca (or of the bodies formerly called "Gallert-cilien," afterwards recognised as "contractile Filamente").

Synopsis of the Genera of Astrolonchida.

I. Subfamily Zygacanthida. Twenty radial spines simple, without apophyses or lateral transverse processes. Spines cylindrical, with circular transverse section, 323. Acanthometron.

Spines compressed, two-edged or lamellar, with elliptical or rhomboidal transverse section, 324. Zygacantha.

Spines quadrangular (prismatic or pyramidal), with four edges, with square transverse section, 325. Acanthonia.

II. Subfamily Phractacanthida. Twenty radial spines provided each with two opposite apophyses (or two longitudinal rows of apophyses). Two apophyses opposite on each spine. Apophyses simple, 326. Lithophyllium.

Apophyses branched, 327. Phractacantha.

Apophyses latticed, 328. Doracantha.

Two opposite longitudinal rows of apophyses (four to eight or more apophyses on each spine, opposite in pairs), 329. Astrolonche.

III. Subfamily Stauracanthida. Twenty radial spines provided each with four crossed apophyses (or four crossed longitudinal rows of apophyses). Four apophyses (in cross form) opposite in pairs on each spine. Apophyses simple, 330. Xiphacantha.

Apophyses branched, 331. Stauracantha.

Apophyses latticed, 332. Phatnacantha.

Four longitudinal rows of apophyses, opposite in pairs in cross form on each spine, 333. Pristacantha.

I. Subfamily Zygacanthida. Twenty radial spines simple, without apophyses or lateral transverse processes.

Spines cylindrical, with circular transverse section,

323. Acanthometron.

Spines compressed, two-edged or lamellar, with elliptical or rhomboidal transverse section,

324. Zygacantha.

Spines quadrangular (prismatic or pyramidal), with four edges, with square transverse section,

325. Acanthonia.

II. Subfamily Phractacanthida. Twenty radial spines provided each with two opposite apophyses (or two longitudinal rows of apophyses).

Two apophyses opposite on each spine.

Apophyses simple,

326. Lithophyllium.

Apophyses branched,

327. Phractacantha.

Apophyses latticed,

328. Doracantha.

Two opposite longitudinal rows of apophyses (four to eight or more apophyses on each spine, opposite in pairs),

329. Astrolonche.

III. Subfamily Stauracanthida. Twenty radial spines provided each with four crossed apophyses (or four crossed longitudinal rows of apophyses).

Four apophyses (in cross form) opposite in pairs on each spine.

Apophyses simple,

330. Xiphacantha.

Apophyses branched,

331. Stauracantha.

Apophyses latticed,

332. Phatnacantha.

Four longitudinal rows of apophyses, opposite in pairs in cross form on each spine,

333. Pristacantha.

Subfamily 1. Zygacanthida, Haeckel.

Definition.—Astrolonchida with twenty simple radial spines, without apophyses or lateral transverse processes.

Genus 323. Acanthometron,^[363] J. Müller, 1855, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 229.

Definition.—Astrolonchida with simple cylindrical or needle-shaped radial spines, without edges and without apophyses; their transverse section is circular.

The genus Acanthometron, with the restricted definition here given, is the most simple form of all Acanthonida, and may be regarded as the common ancestral form not only of this suborder but also of all Acanthophracta, in general of all Icosacantha, or all Acantharia in which twenty radial spines are regularly disposed after the Müllerian law (p. 717). In the wider sense, given originally to Acanthometra by Johannes Müller, its discoverer, this genus comprised all Acantharia constituting here our order "Acanthometra" (Radiolaria without lattice-shell, with radial spines united in the centre). In my Monograph (1862, p. 375) I restricted this genus to those "Acanthometrida" in which twenty simple spines of equal size (and without apophyses) are supported one upon another in the centre, and I separated as Astrolithium those forms in which they are grown together in the centre. But this difference now appears not so important, and I restrict here the genus Acanthometron (not Acanthometra) to those most simple forms in which the simple radial spines are cylindrical or conical, without edges.

Subgenus 1. Acanthometrella, Haeckel.

Definition.—Spines at the central base without leaf-cross, united by the opposed triangular faces of their pyramidal bases, resting one upon another.

1. Acanthometron elasticum, Haeckel.

Acanthometra elastica, Haeckel, 1862, Monogr. d. Radiol., p. 376, Taf. xv. fig. 1, Taf. xviii. fig. 1.

Acanthometra elastica, R. Hertwig, 1879, Organismus d. Radiol., Taf. i. figs. 2, 2a, 2b.

Spines cylindrical, very thin and long, needle-shaped, at the central base four-sided pyramidal, without leaf-cross. Distal apex conical. The spines are very elastic, of nearly equal thickness in their whole length. Central capsule quite pellucid, colourless, with a variable number of yellow pigment-bodies (xanthellæ?).

Dimensions.—Length of the spines 0.3 to 0.6, breadth 0.001 to 0.002.

Habitat.—Cosmopolitan, very common in all warmer seas; Mediterranean, Atlantic, Indian, Pacific, surface.

2. Acanthometron cylindricum, n. sp. (Pl. 130, fig. 2).

Spines cylindrical, thick and long, at the central base thickened with a pear-shaped knob, and with very small central fulcral pyramid, without leaf-cross. Distal apex rounded or truncated. Central capsule opaque, filled with red pigment-bodies.

Dimensions.—Length of the spines 0.4 to 0.8, breadth 0.01 to 0.015.

Habitat.—Central Pacific, Stations 266 to 274, surface.

3. Acanthometron fuscum, J. Müller.

Acanthometra fusca, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 47, Taf. xi. fig. 4.

Acanthometra fusca, Haeckel, 1862, Monogr. d. Radiol, p. 377.

Spines very thin and long, in the proximal half cylindrical, in the distal half conical, gradually thinned towards the simple conical apex. Central base a small four-sided pyramid, without leaf-cross. Central capsule opaque, filled with brown pigment-bodies.

Dimensions.—Length of the spines 0.2 to 0.4, breadth 0.002 to 0.004.

Habitat.—Mediterranean (Cette, Portofino, Messina).

4. Acanthometron bulbosum, Haeckel.

Acanthometra bulbosa, Haeckel, 1862, Monogr. d. Radiol., p. 377, Taf. xv. fig. 2, Taf. xviii fig. 2.

Spines very thin and long, cylindrical, with simple needle-shaped apex; in the basal part conical, towards the centre much thickened, with a short four-sided fulcral pyramid, without leaf-cross. Central capsule opaque, filled with brown pigment-bodies.

Dimensions.—Length of the spines 0.3 to 0.5, breadth in the middle and outer part 0.001, in the basal part 0.008 to 0.012.

Habitat.—Mediterranean; Atlantic, Station 354, surface.

5. Acanthometron dolichoscion, Haeckel (Pl. 129, figs. 6-8).

Acanthometra dolichoscia, Haeckel, 1862, Monogr. d. Radiol., p. 377, Taf. xviii. figs. 3a, 3b.

Spines very long and thin, cylindrical, in the thickened outer half about twice as thick as in the thin inner half, at the simple apex compressed, two-edged. Central base little thickened, with a small four-sided pyramid, without leaf-cross. Central capsule transparent, whitish or yellowish.

Dimensions.—Length of the spines 0.6 to 0.8, breadth in the proximal part 0.004, in the distal part 0.008.

Habitat.—Mediterranean (Messina, Portofino), surface.

6. Acanthometron conicum, n. sp.

Spines short and thick, conical, gradually thinner from the thick conical base towards the simple apex. Central part of the base with a large fulcral pyramid, but without leaf-cross. Central capsule transparent, colourless (?).

Dimensions.—Length of the spines 0.08 to 0.12, basal breadth 0.02.

Habitat.—North Pacific, Station 244, surface.

7. Acanthometron pellucidum, J. Müller.

Acanthometra pellucida, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 47, Taf. xi. figs. 1-3.

Acanthometra pellucida, Haeckel, 1862, Monogr. d. Radiol., p. 378.

Spines cylindrical, very thin and long, needle-shaped, nearly of equal thickness in their whole length. Central base with a small fulcral pyramid, without leaf-cross. Distal apex bifid or bifurcate, with two thin parallel teeth. Central capsule pellucid, colourless, with yellow pigment-bodies. (Differs from Acanthometron elasticum mainly in the bifid apex.)

Dimensions.—Length of the spine 0.1 to 0.3, breadth 0.002 to 0.004.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface.

8. Acanthometron wageneri, Haeckel.

Acanthometra wageneri, Haeckel, 1862, Monogr. d. Radiol., p. 378.

Spines cylindrical, in the thickened outer half about twice as broad as in the thin inner half. Central base a little thickened, with large fulcral pyramid, but without leaf-cross. Distal apex bifid, with two divergent, often denticulated teeth. Central capsule pellucid, with yellow pigment-bodies. (Differs from Acanthometron dolichoscion mainly in the bifid apex.)

Dimensions.—Length of the spines 0.2 to 0.5, breadth in the inner part 0.003, in the outer 0.006.

Habitat.—Adriatic Sea; Triest, Wagner; Corfu, Haeckel, surface.

Subgenus 2. Phyllostaurus, Haeckel, 1862, Monogr. d. Radiol., p. 381.

Definition.—Spines at the central base with a broad leaf-cross, composed of four prominent triangular lamellæ; the meeting edges of the neighbouring lamellæ are propped one upon another in such a manner that there are formed twenty-two hollow pyramidal spaces or compartments (compare p. 721).

9. Acanthometron siculum, Haeckel.

Acanthometra sicula, Haeckel, 1862, Monogr. d. Radiol., p. 382, Taf. xvii. figs. 1, 2; Taf. xviii. fig. 8.

Spines elongate, conical, tapering gradually from the thick base towards the simple distal apex. Conical circular base supported by a basal leaf-cross of double the breadth. Central capsule yellowish-brown, opaque.

Dimensions.—Length of the spines 0.3 to 0.4, basal breadth 0.008 to 0.012, leaf-cross 0.024.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface.

10. Acanthometron catervatum, Haeckel.

Acanthometra brevispina, Haeckel, 1862, Monogr. d. Radiol., p. 382, Taf. xv. fig. 5, Taf. xviii. fig. 9.

Spines cylindrical, nearly of equal breadth throughout their whole length. Apex either simple, conical, or bifid. Base with a large leaf-cross, four to six times as broad as the spine itself. Central capsule transparent, yellow. The Atlantic specimens have much longer spines than those figured from the Mediterranean, but are otherwise not different. Therefore I have changed the inconvenient name brevispinum into catervatum.

Dimensions.—Length of the spines 0.1 to 0.3, breadth 0.005 to 0.008; basal leaf-cross 0.02 to 0.032.

Habitat.—Mediterranean (Messina); North Atlantic, Station 354, Gulf Stream (Færöe Channel) in great abundance, John Murray, surface.

Subgenus 3. Astrolithium, Haeckel, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 810.

Definition.—Spines in the basal part grown perfectly together, so that the whole skeleton forms a single piece of acanthin; a star with twenty equal rays.

11. Acanthometron bulbiferum, n. sp. (Pl. 130, fig. 1).

Astrolithium bulbiferum, Haeckel, 1881, Prodromus, p. 466.

Spines needle-shaped, cylindrical, very thin and long, with simple apex; suddenly thickened and forming a broad regular cone at the central base; all twenty conical bulbs have their broad bases grown together and forming a central icosahedron of acanthin; from its twenty faces arise the circular bases of the cones in regular disposition.

Dimensions.—Length of the spines 0.2 to 0.5, breadth 0.001 to 0.003; height of the basal cones 0.06, basal breadth of them 0.02.

Habitat.—Central Pacific, Stations 265 to 274, surface.

12. Acanthometron bifidum, Haeckel.

Astrolithium bifidum, Haeckel, 1862, Monogr. d. Radiol., p. 400, Taf. xx. fig. 5.

Spines needle-shaped, cylindrical, of equal breadth in their whole length. Distal apex bifid, with two parallel straight teeth. Central bases of all twenty spines grown perfectly together and forming a central sphere of acanthin. Central capsule brown opaque.

Dimensions.—Length of the spines 0.2, breadth 0.003.

Habitat.—Mediterranean (Messina), Haeckel, surface.

Genus 324. Zygacantha,^[364] J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 51.

Definition.—Astrolonchida with simple, compressed, and two-edged radial spines, without apophyses; their transverse section is elliptical or rhomboidal.

The genus Zygacantha comprised in the original definition of J. Müller only a single species, Zygacantha furcata, distinguished from the other Acanthometra by forked spines with two long parallel teeth. It seems now advisable to unite in this genus all those Astrolonchida in which the simple spines are two-edged, compressed, or leaf-shaped. The term Zygacantha may be conceived as the general expression of the important fact, that in all Icosacantha the twenty spines are opposite in pairs.

Subgenus 1. Zygacantharium, Haeckel.

Definition.—Spines at the central base without leaf-cross and without hollow pyramidal compartments, united by the opposed triangular faces of their pyramidal bases, resting one upon another.

1. Zygacantha lanceolata, Haeckel.

Acanthometra lanceolata, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 48, Taf. xi. fig. 12.

Acanthometra lanceolata, Haeckel, 1862, Monogr. d. Radiol., p. 380.

Spines lanceolate, from the broader middle part equally thinned towards the two ends. Apex simple. Base pyramidal, without leaf-cross. Each flat lamellar spine exhibits an elevated middle rib (like a lanceolate leaf), and is therefore compressed quadrangular.

Dimensions.—Length of the spines 0.1 to 0.15, greatest breadth (in the width) 0.03 to 0.04.

Habitat.—Mediterranean (Saint Tropez, French shore), J. Müller; North Atlantic (Canary Islands), Haeckel, surface.

2. Zygacantha costata, n. sp.

Spines compressed, two-edged, linear, of nearly equal breadth in their whole length. Apex truncate. Base pyramidal, without leaf-cross. Each flat lamellar spine exhibits an elevated middle rib, which in the distal half is cleft into two divergent rods ending in the corners of the truncated apex. (Similar to Zygacantha dicopa, but with broader free spines, which are not grown together in the centre.)

Dimensions.—Length of the spines 0.1 to 0.15, breadth 0.02.

Habitat.—Central Pacific, Stations 265 to 274, surface.

3. Zygacantha compressa, Haeckel.

Acanthometra compressa, Haeckel, 1862, Monogr. d. Radiol., p. 378, Taf. xviii. figs. 4a, 4b.

Spines in the proximal half compressed, broad lanceolate, four to six times as broad as in the thin cylindrical distal half. Apex simple or short bifid. Base pyramidal, thickened, without leaf-cross. No middle rib. Central capsule opaque, yellowish or reddish-brown.

Dimensions.—Length of the spines 0.3 to 0.5, breadth of the inner half 0.015 to 0.02, of the outer half 0.002 to 0.003.

Habitat.—Mediterranean (Messina); Atlantic (Canary Islands), Station 352, surface.

4. Zygacantha furcata, J. Müller.

Zygacantha furcata, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 51, Taf. ix. fig. 6.

Acanthometra furcata, J. Müller, 1856, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 499.

Acanthometra furcata, Haeckel, 1862, Monogr. d. Radiol., p. 380.

Spines compressed, fork-shaped, divided by two thickened knots into three sections of nearly equal length; middle section broader than the proximal, but smaller than the distal section, which is cleft by a deep fissure into two long parallel straight branches. Base pyramidal, without leaf-cross. Central capsule purple, with yellow bodies.

Dimensions.—Length of the spines 0.1 to 0.15, basal breadth 0.003 to 0.005, distal breadth 0.02 to 0.03.

Habitat.—Mediterranean (Cette). J. Müller, surface.

Subgenus 2. Zygacanthidium, Haeckel.

Definition.—Spines at the central base with a cross of four prominent leaves; the meeting edges of the neighbouring lamellæ so rest one upon another that twenty-two hollow pyramidal compartments are formed (compare p. 721).

5. Zygacantha dichotoma, Haeckel.

Zygacantha dichotoma, Haeckel, 1862, Monogr. d. Radiol., p. 381.

Acanthometra dichotoma, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 50, Taf. ix. fig. 5.

Spines compressed, pincer-shaped, cleft nearly throughout their whole length into two thin parallel straight rods or fork branches, which are united only in their middle by a narrow bridge, and at their central base by the pyramidal small central apex; above this pyramid each rod is divided into two broad triangular leaves, forming a large basal leaf-cross. Central capsule purple, with yellow bodies.

Dimensions.—Length of the spines 0.1 to 0.15, breadth 0.01 to 0.02.

Habitat.—Mediterranean (Nice), J. Müller, surface.

6. Zygacantha complanata, n. sp.

Spines compressed, two-edged, linear, of equal breadth throughout their whole length. Apex bifid. Base thickened, with a large leaf-cross. (Similar to Amphilonche complanata, but different in the equal size of all twenty spines, which are somewhat broader.)

Dimensions.—Length of the spines 0.12 to 0.18, breadth 0.002.

Habitat.—North Pacific, Station 244, surface.

7. Zygacantha semicompressa, Haeckel.

Acanthometra hemicompressa, Car, 1884, Zool. Anzeiger, p. 94, with woodcut.

Spines in the proximal half compressed, two-edged, linear, three to four times as broad as in the needle-shaped cylindrical distal half. Apex simple. Base pyramidal, with a small leaf-cross. Central capsule transparent.

Dimensions.—Length of the spines 0.1 to 0.2, breadth in the proximal half 0.02, in the distal half 0.006.

Habitat.—Mediterranean (Genoa), Haeckel; Adriatic Sea (Trieste), Car, surface.

8. Zygacantha foliacea, n. sp.

Spines lanceolate, tapering from the broader middle part towards the two ends, with two dentated or serrated edges and a prominent middle rib. Apex simple. Base with a small leaf-cross. The spines like the leaves of Agave americana.

Dimensions.—Length of the spines 0.15 to 0.2, middle breadth 0.02 to 0.03.

Habitat.—Indian Ocean, near the Island of Socotra, Haeckel.

Subgenus 3. Zygacanthonium, Haeckel.

Definition.—Spines in the centre grown perfectly together and forming a single star-shaped piece of acanthin.

9. Zygacantha dicopa, Haeckel.

Astrolithium dicopum, Haeckel, 1862, Monogr. d. Radiol., p. 400, Taf. xx. figs. 3, 4.

Spines compressed, two-edged, linear, of nearly equal breadth throughout their whole length, with a prominent middle rib, which in the distal third is cleft into two divergent teeth, ending in the two corners of the broad, obliquely truncated apex. All twenty spines with their central bases grown together and forming one single piece of acanthin—a star with twenty rays.

Dimensions.—Length of the spines 0.12 to 0.2, breadth 0.01 to 0.02.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, many Stations, surface.

Genus 325. Acanthonia,^[365] Haeckel, 1881, Prodromus, p. 465.

Definition.—Astrolonchida with simple, four-edged, prismatic or pyramidal radial spines, without apophyses; their transverse section is square.

The genus Acanthonia comprises all those Astrolonchida (formerly united with Acanthometron) in which the simple spines either in their whole length or in the greatest part of it are four-edged, with square transverse section. They are sometimes more prismatic (with equal breadth), at other times more pyramidal (with decreasing breadth towards the distal apex). If Acanthometron be the common simple ancestral form of the Acanthonida, then the two-edged Zygacantha, and the four-edged Acanthonia may be regarded as two divergent main lines arising from it; the former leading to the Phractacanthida and Diporaspida, the latter leading to the Stauracanthida and Tessaraspida.

Subgenus 1. Acanthonarium, Haeckel.

Definition.—Spines at the central base, without leaf-cross and without hollow pyramidal compartments, united by the triangular faces of their pyramidal bases, resting one upon another.

1. Acanthonia tetracopa, Haeckel (Pl. 129, figs. 9-11).

Acanthometra tetracopa, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 47, Taf. vii. figs. 3 to 5, Taf. xi. fig. 5.

Acanthometra tetracopa, Haeckel, 1862, Monogr. d. Radiol., p. 379, Taf. xviii. fig. 5.

Spines quadrangular prismatic, with prominent lamellar edges, of equal breadth in their whole length. Base pyramidal, without leaf-cross. Apex truncated, or with four short teeth (sometimes only two teeth). Central capsule opaque, brown or yellow.

Dimensions.—Length of the spines 0.15 to 0.2, breadth 0.01 to 0.012.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific, many Stations, surface.

2. Acanthonia prismatica, n. sp.

Spines quadrangular prismatic, without prominent edges, with four plane lateral faces, of equal breadth throughout their whole length. Base pyramidal, without leaf-cross. Apex truncate, with square apical face.

Dimensions.—Length of the spines 0.1 to 0.16, breadth 0.005 to 0.008.

Habitat.—Antarctic Ocean, Station 154, surface.

3. Acanthonia denticulata, n. sp.

Spines quadrangular prismatic, with prominent elegantly denticulated edges, of equal breadth throughout their whole length. Both ends pyramidal, without leaf-cross. (Similar to Amphilonche denticulata, but with the twenty spines all equal, and with spherical central capsule.)

Dimensions.—Length of the spines 0.12 to 0.16, breadth 0.008.

Habitat.—Central Pacific, Stations 266 to 274, surface.

4. Acanthonia mülleri, Haeckel.

Acanthometra mülleri, Haeckel, 1862, Monogr. d. Radiol., p. 379, Taf. xv. fig. 3, Taf. xviii. fig. 6.

Spines quadrangular, nearly prismatic, but tapering gradually from the pyramidal base towards the distal bifid apex, which bears two thin, parallel, or little divergent teeth; four edges elegantly denticulated; base without leaf-cross. Central capsule yellowish or reddish.

Dimensions.—Length of the spines 0.12 to 0.18, basal breadth 0.005.

Habitat.—Mediterranean (Messina); Atlantic, Stations 348 to 352, surface.

5. Acanthonia fragilis, Haeckel.

Acanthometra fragilis, Haeckel, 1862, Monogr. d. Radiol., p. 380, Taf. xv. fig. 4, Taf. xviii. fig. 7.

Spines quadrangular, nearly prismatic, but gradually thickened from the pyramidal base towards the distal truncated end; four edges regularly denticulated; base without leaf-cross. Central capsule opaque.

Dimensions.—Length of the spines 0.5 and more, basal breadth 0.002 to 0.003, distal breadth 0.008 to 0.016 or more.

Habitat.—Mediterranean (Messina); North Atlantic, Station 354, surface.

6. Acanthonia convexa, n. sp.

Spines quadrangular, with four broad lamellar, convex, prominent edges, which from the broader middle part are thinned towards the two emarginated ends; each end with a small quadrangular pyramid, without leaf-cross. All the twenty spines of this species exhibit nearly the same form, which is seen in the caudal or posterior (geotomical) spine of Amphilonche anomala (Monogr. d. Radiol., Taf. xviii. fig. 23, b).

Dimensions.—Length of the spines 0.12 to 0.16, breadth in the middle part 0.02 to 0.025.

Habitat.—North Pacific, Station 244, surface.

7. Acanthonia concava, n. sp.

Spines quadrangular, with four broad lamellar, concave, prominent edges, which from the smaller middle part are broadened towards the two emarginated ends; each end with a small quadrangular pyramid, without leaf-cross. All the twenty spines of this species exhibit nearly the same form, which is seen in the frontal or anterior (hydrotomical) spine of Amphilonche anomala (Monogr. d. Radiol., Taf. xviii. fig. 23, a).

Dimensions.—Length of the spines 0.15, breadth in the middle part 0.012, on both ends 0.025.

Habitat.—South Pacific, Station 288, surface.

8. Acanthonia quadrangula, n. sp.

Spines quadrangular prismatic, with smooth prominent straight edges, but of very different breadth in both halves; the inner or proximal half (inside the central capsule) twice to four times as broad as the outer or distal half (outside the central capsule); both ends suddenly separated by a constriction, in which is inserted the membrane of the capsule. The latter is pellucid, with a number of yellow bodies. Apex of the spines simple, base pyramidal, without leaf-cross.

Dimensions.—Length of the spines 0.2 to 0.4, breadth of the basal half 0.01, of the distal half 0.002 to 0.004.

Habitat.—Atlantic, Canary Islands, Azores, Station 354, surface.

Subgenus 2. Acanthonidium, Haeckel.

Definition.—Spines at the central base with a broad leaf-cross, composed of four prominent triangular lamellæ; the meeting edges of these crossed lamellæ between every three or four neighbouring spines so rest one upon another that twenty-two hollow pyramidal compartments are formed (compare p. 721).

9. Acanthonia echinoides, Haeckel.

Acanthometra echinoides, Claparède et Lachmann, 1858, Études sur les Infusoires et les Rhizopodes, &c., p. 459, pl. xxiii. figs. 1-5.

Acanthometra echinoides, Haeckel, 1862, Monogr. d. Radiol., p. 383.

Spines quadrangular prismatic without prominent edges, with four plane lateral faces, of equal breadth throughout their whole length. Central base three to four times as broad, with large leaf-cross. Apex truncated or bifid (sometimes with four short teeth).

Dimensions.—Length of the spines 0.3 to 0.5, breadth 0.004 to 0.008; leaf-cross 0.02 to 0.03.

Habitat.—North Atlantic, west coast of Norway (Claparède et Lachmann); Færöe Channel (Gulf Stream), John Murray, surface.

10. Acanthonia claparedei, Haeckel.

Acanthometra claparedei, Haeckel, 1862, Monogr. d. Radiol., p. 383, Taf. xviii. fig. 12.

Spines quadrangular prismatic, with four prominent lamellar edges, of equal breadth throughout their whole length. Apex bifid. Central base twice as broad, with large leaf-cross. Central capsule opaque, reddish-brown.

Dimensions.—Length of the spines 0.6, breadth 0.016; leaf-cross 0.032.

Habitat.—Mediterranean (Messina); Atlantic, Station 347, surface.

11. Acanthonia cuspidata, Haeckel.

Acanthometra cuspidata, Haeckel, 1862, Monogr. d. Radiol., p. 383, Taf. xviii. fig. 11.

Spines quadrangular pyramidal, with four prominent lamellar edges, tapering gradually from the broad base towards the simple distal apex. Basal leaf-cross as broad, with a small central pyramid.

Dimensions.—Length of the spines 0.5, basal breadth 0.012; leaf-cross 0.025.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface.

12. Acanthonia quadrifolia, Haeckel.

Acanthometra quadrifolia, Haeckel, 1862, Monogr. d. Radiol., p. 382, Taf. xviii. fig. 10.

Spines in the basal part quadrangular pyramidal, with four prominent lamellar edges and a large basal leaf-cross; in the distal part three to six times as long, cylindrical, of equal breadth. Apex simple, conical, or bifid. Central capsule yellow or whitish. This common species differs from the nearly allied Acanthometron catervatum mainly by the strong development of the large basal leaves or wings.

Dimensions.—Length of the spines 0.3 to 0.5, breadth in the distal half 0.002, in the basal part 0.02.

Habitat.—Mediterranean; North Atlantic, Stations 252 to 254; Færöe Channel, Gulf Stream, in enormous numbers, John Murray, surface and at various depths.

13. Acanthonia diplopyramis, n. sp.

Spines formed like a quadrangular double pyramid or an irregular octahedron; the basal leaf-cross being extremely developed, with four very large and thin lamellar leaves; the basal halves of the twenty double pyramids are united by the meeting edges of those leaves, while their distal halves are free, with simple apices. Therefore the four triangular leaves of each spine are equally thinned from the middle towards the two ends.

Dimensions.—Length of the spines 0.1 to 0.2, greatest breadth 0.04 to 0.08.

Habitat.—Central Pacific, Stations 266 to 274, surface.

14. Acanthonia multispina, Haeckel.

Acanthometra multispina, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 47, Taf. vii. figs. 6-9.

Acanthometra multispina, Haeckel, 1862, Monogr. d. Radiol., p. 384.

Spines quadrangular, with four lamellar prominent edges, in the proximal half nearly prismatic, in the distal half pyramidal; both halves separated by a prominent short tooth on each edge. Here in the middle part the breadth (including the four teeth) is equal to the basal leaf-cross. Central capsule opaque, brown.

Dimensions.—Length of the spines 0.2 to 0.3, greatest breadth 0.01 to 0.02.

Habitat.—Mediterranean (Messina); Tropical Atlantic, Station 348.

15. Acanthonia serrulata, n. sp.

Spines quadrangular pyramidal, with simple apex; the large basal leaf-cross nearly half as long as the prolonged distal part. The four prominent triangular edges of the latter are very thin and broad lamellæ, finely dentate or serrate.

Dimensions.—Length of the spines 0.2 to 0.3, greatest breadth 0.05 to 0.06.

Habitat.—Central Pacific, Station 272, surface.

Subgenus 3. Acantholithium, Haeckel.

Definition.—Spines in the basal part grown together, so that the whole skeleton is not composed of twenty separated pieces, but represents a single piece of acanthin—a star with twenty equal rays.

16. Acanthonia stellata, n. sp.

Spines quadrangular pyramidal, with simple distal apex, in the basal part grown perfectly together, so that the whole skeleton forms a single piece of acanthin—a starlet with twenty equal rays. The free pyramidal part of each spine is twice to four times as long as the basal part.

Dimensions.—Length of the spines 0.1 to 0.15, greatest breadth (on the surface of the central solid sphere) 0.02 to 0.04.

Habitat.—Central Pacific, Station 271, surface.

Subfamily 2. Phractacanthida, Haeckel.

Definition.—Astrolonchida with twenty radial spines, each of which bears two opposite apophyses or lateral transverse processes; sometimes two longitudinal rows of opposite apophyses.

Genus 326. Lithophyllium,^[366] J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 52.

Definition.—Astrolonchida with two simple, not branched, opposite apophyses on each radial spine.

The genus Lithophyllium was founded by J. Müller for a single species (Lithophyllium foliosum), which we also here retain as the type of the genus. It is the first observed Astrolonchid, which bears two opposite lateral apophyses on each spine, and may therefore be regarded as the ancestral form of the subfamily Phractacanthida. The two opposite apophyses are here simple, whilst in the other genera of the subfamily they are branched or multiplied.

1. Lithophyllium cruciatum, Haeckel.

Acanthometra cruciata, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 49, Taf xi. fig. 11.

Xiphacantha cruciata, Haeckel, 1862, Monogr. d. Radiol., p. 385, Taf. xviii. fig. 13.

Spines cylindrical, very thin, crossed perpendicularly in the distal third by a thin transverse beam; both lateral rods of the cross have the same length as the distal end. Apex simple. Base pyramidal, without leaf-cross.

Dimensions.—Length of the spines 0.05 to 0.15, breadth 0.001 to 0.002.

Habitat.—Mediterranean, Atlantic, Stations 352 to 354, &c., surface.

2. Lithophyllium gladiatum, n. sp. (Pl. 130, fig. 3).

Spines compressed, sword-shaped, two-edged, crossed perpendicularly in the middle part by a broad, somewhat curved transverse beam; both lateral rods of the cross are triangular compressed, and have the same length as the proximal end. Apex simple. Base pyramidal, without leaf-cross.

Dimensions.—Length of the spines 0.2 to 0.25, breadth 0.01.

Habitat.—North Pacific, Station 241, surface.

3. Lithophyllium condylatum, n. sp.

Spines compressed quadrangular, tapering from the broader middle towards the two ends; from the middle part arise two opposite perpendicular apophyses, which bear on the rounded end a thickened condyle. Apex simple. Base pyramidal, with a small leaf-cross.

Dimensions.—Length of the spines 0.1 to 0.12, breadth in the middle part 0.02.

Habitat.—Central Pacific, Station 266 to 272, surface.

4. Lithophyllium foliosum, J. Müller.

Lithophyllium foliosum, J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 52, Taf. xi. figs. 6-10.

Xiphacantha foliosa, Haeckel, 1862, Monogr. d. Radiol., p. 385.

Spines lanceolate, tapering from the broader middle towards the two ends. From the middle part or from the outer third arise two opposite triangular apophyses, which are not perpendicular to the axis of the spine, but form an acute angle with its distal part. Therefore each spine represents a broad leaf with a middle rib and with three lobes or truncated teeth. The distal apex is distinguished by a violet colour. Base pyramidal, without leaf-cross. Central capsule yellow.

Dimensions.—Length of the spines 0.05 to 0.1, breadth 0.02 to 0.03.

Habitat.—Mediterranean, French shore, Saint Tropez, J. Müller.

Genus 327. Phractacantha,^[367] Haeckel, 1881, Prodromus, p. 465.

Definition.—Astrolonchida with two branched, but not latticed, opposite apophyses on each radial spine.

The genus Phractacantha differs from its ancestral form, the preceding Lithophyllium by the ramification of the apophyses, which are either forked or bear lateral branches. If the prolonged fork-branches of the neighbouring spines meet and form a lattice-shell, this genus passes over into Phractaspis, the common ancestral form of the Diporaspida.

1. Phractacantha bifurca, n. sp.

Spines cylindrical, thin, of equal breadth throughout their whole length. Apex simple. Base pyramidal, without leaf-cross. From the outer third arise two opposite, thin, forked apophyses; ends of the fork branches acute.

Dimensions.—Length of the spines 0.1, breadth 0.006.

Habitat.—Central Pacific, Station 274, surface.

2. Phractacantha bipennis, n. sp.

Spines compressed, two-edged, tapering from the broader middle towards the two ends. Apex simple. Base pyramidal, without leaf-cross. From the middle arise two opposite broad, forked apophyses; ends of the fork-branches broad and obtuse.

Dimensions.—Length of the spines 0.15, breadth in the middle 0.012.

Habitat.—Central Pacific, Station 266, surface.

Genus 328. Doracantha,^[368] Haeckel, 1881, Prodromus, p. 465.

Definition.—Astrolonchida with two latticed or fenestrated apophyses on each radial spine.

The genus Doracantha arises from the foregoing Phractacantha by union of the fork-branches of the apophyses on each spine. By this concrescence is formed a polygonal or roundish plate with two pores, pierced by the radial spine between them. Doracantha may be regarded as a Dorataspis, in which the twenty small fenestrated plates have not become united.

1. Doracantha dorataspis, n. sp.

Spines compressed, two-edged, tapering from the broader middle towards the two ends. Apex simple. Base pyramidal, without leaf-cross. From the middle part of each spine arise two opposite forked apophyses; the neighbouring fork-branches are recurved and united in the tangential plane; so that each spine bears a plate or shield with two elliptical pores; the margin of the roundish plate bears a variable number of short teeth.

Dimensions.—Length of the spines 0.2, breadth 0.02.

Habitat.—Central Pacific, Station 271, surface.

Genus 329. Astrolonche,^[369] Haeckel, 1881, Prodromus, p. 465.