Note 5 — Chapter VIII · DEVELOPMENTAL HISTORY OF EDRIOPHTHALMA.

In the young of Hyperia galba Spence Bate did not find any of the abdominal feet, or the last two pairs of thoracic feet, but this very remarkable statement required confirmation the more because he examined these minute animals only in the dried state. Subsequently I had the wished-for opportunity of tracing the development of a Hyperia which is not uncommon upon Ctenophora, especially Beroë gilva, Eschsch. The youngest larva from the brood-pouch of the mother already possess the whole of the thoracic feet; on the other hand, like Spence Bate, I cannot find those of the abdomen. At first simple enough, all these feet soon become converted, like the anterior feet, into richly denticulated prehensile feet, and indeed of three different forms, the anterior feet (Fig. 44) the two following pairs (Fig. 45) and finally the three last pairs (Fig. 46) being similarly constructed and different from the rest. In this form the feet remain for a very long time, whilst the abdominal appendages grow into powerful natatory organs, and the eyes, which at first seemed to me to be wanting, into large hemispheres. In the transition to the form of the adult animal the last three pairs of feet (Fig. 49) especially undergo a considerable change. The difference between the two sexes is considerable; the females are distinguished by a very broad thorax, and the males (Lestrigonus) by very long antennæ, of which the anterior bear an unusual abundance of olfactory filaments. Their youngest larvæ of course cannot swim; they are helpless little animals which firmly cling especially to the swimming laminæ of their host; the adult Hyperiæ, which are not unfrequently met with free in the sea, are, as is well known, the most admirable swimmers in their order. (“Il nage avec une rapidité extrême,” says Van Beneden of H. Latreillii M.-Edw.) The transformation of the Hyperiæ is evidently to be regarded as acquired and not inherited, that is to say the late appearance of the abdominal appendages and the peculiar structure of the feet in the young are not to be brought into unison with the historical development of the Amphipoda, but to be placed to the account of the parasitic mode of life of the young. As in Brachyscelus, free locomotion has been continued to the adult and not to the young, contrary to the usual method among parasites. Still more remarkable is a similar circumstance in Caligus, among the parasitic Copepoda. The young animal, described by Burmeister as a peculiar genus, Chalimus, lies at anchor upon a fish by means of a cable springing from its forehead, and having its extremity firmly seated in the skin of the fish. When sexual maturity is attained, the cable is cut, and the adult Caligi, which are admirable swimmers, are not unfrequently captured swimming freely in the sea. (See ‘Archiv. für Naturgeschichte’ 1852, I. p. 91).