III. Genera with stemmata and ocelli.

In a little group that has retained larval or ancestral characters during a great part of the palæozoic period, the genus Harpes stands as a type. It has ranged from the oldest Lower Silurian, if we join the related Harpides, to the middle Devonian. From near the top of the glabella, though not so much forward as the facial ridge of the blind trilobites (Olenus, Liostracus etc.) a straight ridge of much varying length stands out on both sides and at its extremity two or three globular stemmata with glossy surface lie encased. Probably this ridge has the same origin as in the Olenidæ, the more so, as there are indications of an extensive circulatory system. On plate IV fig. 18-19 the right hand ocelli of Harpes vittatus Barr. from Lochkow, Bohemia, are represented. They are two, lying isolated near each other, quite globular with circular outline, smooth and glossy as to exhibit a shining surface. Their size is 0,4 mm in diameter. Being cut vertically in the direction of the longitudinal axis of the head they resemble elongated hemispheres, convex on the exterior surface, slightly concave on the interior. The test of the head lies between them as a saddle and covers them only partially and on the outer sides they lie with their margins encased in the head shield. Seen in thin sections of the right lens and magnified the whitish mass is traversed by vertical, blacker streaks, standing somewhat radiating towards the sides and cancellate. The other lens has a horizontal row of black dots. All this is probably not of any structural value, only due to later changes.

The remarkable genus Harpides from the lowest Lower Silurian belongs also to this group and has beside the fixed peduncle a peculiar elongated ridge going from the eyes to the lateral margins of the head[33], a ridge which is also present in some of the true Harpes.

As Beecher has shown[34] the larva of Trinucleus possesses quite the same transverse ridge with intumescent eyelike extremities, and although the smallness of the specimens has not permitted to ascertain the presence of a true eye, it may be apposite to suppose it on the homology with the eyes of Harpes, a genus with which Trinucleus is related. But as well known, in the adult Trinuclei there is no trace of these ocular ridges nor of real eyes so that Beecher in his paper »Blind Trilobites»[35] numerates Trinucleus amongst these. Out of the nine Scandinavian species of Trinucleus no less than seven have a well marked little tubercle on each side of the glabella placed exactly on the same spot where the larval Trinuclei had their much larger eyes placed. In this case it may be allowed to suppose that the tubercles are the direct successors of the larval eye and that they are true ocelli. Reede seems to be willing to regard them as possessing a visual function.[36] Beecher[37] holds the eye nodules in the larva and the ocelli in the adult to be identical.

But, as Barrande has shown,[38] there is a certain species of Trinucleus, the larva of which wants a facial ridge and eyes, as there also are several adult forms without ocelli. These have remained on a much ancestral stage, while the larva with eyes are more highly developed in such species, where the adult have been subject to a retrograde development. Ampyx and Dionide, though completely blind, evidently belong to this group, and once, as is to be hoped, larval forms may be discovered showing their development. In a certain way Arethusina shows characters proper for this group, in having the straight ocular ridge, quite as in Harpes, but eyes of the reticulate type and probably prismatic. It thus like Harpes conserved an ancestral characteristic long periods since it had disappeared in most of the other genera.

Of the groups, in which Joh:s Müller[39] long ago classified the Crustacean eyes, his second »Hauptgruppe» (»Aggregate von einfachen linsenhaften Augen») and the fourth »zusammengesetzten Augen ... facettirte Hornhaut» the former corresponds with my third and the latter with my two first divisions. In so far as the cornea and its facets or lenses are to be regarded, there is the greatest analogy with the Isopoda. In vertical sections of Sphæroma we have the same sort of elongated, flattened biconvex lenses as in Sphærophthalmus and others. Since Grenacher and Exner and others have published their excellent works on the eyes of the Arthropoda, there can be no foundation for speaking of the resemblance of the trilobitic eye with that of Limulus, as this genus stands completely isolated amongst all Arthropoda in that respect. There is, as stated above, a certain resemblance between the cornea of Peltura and that of Limulus, but this is not yet ripe for a discussion. Nor is there any evidence for correlating the eyes of the trilobites with the eyes of the Phyllopoda. Bernard thinks that the so called eye of the Paradoxidæ has been formed upon the same plan as that of Apus. There is nothing to prove this hypothesis that the facial ridge or any part of it ever had been a visual organ, and the evidence at hand rather tends in a contrary direction.

There are signs of long physiological and anatomical efforts to prepare the development of the eyes on the free cheek, as revealed through the long series of blind trilobites. A system of radiating blood vessels, similar to those described above as covering the inside of the head in some older genera, all issuing from the scallop in the free cheek, where later the eye had to find its place, have left their stamp, their mark on the surface of the free cheek. They attest the great vital activity which was so intense at the point were the eye was to be formed. We give the figures of two such cheeks of different types. One from Parabolina spinulosa (pl. V fig. 31) is the more common, where six or more isolated trunks radiate from the semilunar ridge round the indenture and subdivide in branchlets which cease near the lateral margin of the cheek. It may be that it is an annular vessel near the indenture that feeds them all and that this probably is in connection with the great central circulatory system. In Olenus (pl. V fig. 29) the vessels are partly anastomosing and form a reticulate system and they are studded with minute wartlets. Another sign, which may be taken as a preparation, is the elevated rim around the scallop, which is so prominent in several of the Cambrian genera, but which does not embrace any facet bearing cornea..

From what has been stated above the following conclusions have been arrived at.

1. The plurality of the genera living during the Cambrian period were blind and it was first at the close of that period, in the Olenus schists, that genera with real visual organs appeared. There may have been oculate trilobites earlier, as Solenopleura, but we know nothing of their eyes.

2. The primordial glabellar pleuron which was metamorphosed into a facial ridge is no visual organ. It is in the Olenidæ nothing but the elevated line made in the test by the subjacent main trunk of the circulatory system. It swells out in a node, »palpebral lobe», but not before the facial suture has been formed. In the genera where there is no facial suture, there is no node. In the Paradoxidæ where the ridge is of a different origin, there is no node, though there is a suture.

The four types of eyes in the trilobites have probably succeeded one another in the following chronological order:

1) with stemmata or ocelli; 2) biconvex or lentiform; 3) prismatic; 4) aggregate. The oldest known representatives for each type are for 1) Harpides rugosus in the Ceratopyge limestone of the Lower Silur., for 2) Eurycare, in the Cambrian Olenid schists, division 2, for 3) Megalaspis, in the Ceratopyge limestone of the Lower Silurian, for 4) Phacops in the Lower gray Orthoceratite limestone. The eyes of the trilobites show the greatest conformity with those of the recent Isopoda.

The most perfect eyes amongst all the trilobitic eyes may be those of the Phacopidæ, which are also geologically the youngest, the least developed again those of the Proetidæ or rather of the Bumasti. The great thickness of the cornea in these must have weakened their power of vision and they had probably only a faint perception of light.