CHAPTER XXVI

TRIBE VI.-EPICARIDEA

THIS tribe consists of Crustacea which in the adult state are parasitic upon other Crustacea, to which allusion is made in the tribal name signifying 'dwellers upon shrimps.' The females become degraded in form and often very unsymmetrical, while the males, much smaller and symmetrical, are often free, but usually do not quit their partners.

The notion entertained of old by the French fisher-folk, that the Bopyri in the prawn were young flat-fish, received scientific support from M. Deslandes in 1722, but in 1772 was disproved by M. Fougeroux de Bondaroy. For some sixty or seventy years after his time the knowledge of the group was but slowly advanced. Remarkable forms were obscurely described by Cavolini, Montagu, and Risso. Others were made known later on with clearer definition by H. Rathke and Kröyer. Duvernoy and Dana contributed new genera; and by degrees the tribe both gathered volume and evoked attention. During the last thirty years Fritz Müller, R. Kossmann, Paul Fraisse, and various other writers of eminence have thrown light upon the subject from several points of view, and in the latter part of that period the labours of MM. Giard and Bonnier have introduced order and clearness into its arrangement. The writings of these last-named observers will not soon or easily be superseded as the leading authorities on this tribe. They ascribe to it seven families, the Microniscidæ, Cyproniscidæ, Dajidæ, Cabiropsida, Cryptoniscidæ, Entoniscidæ, and Bopyridæ, for which they propose the following phylogenetic table:

The Epicaridea are all, strictly speaking, external parasites, although according to position on their host they may be classified as abdominal, branchial, or visceral. In those which inhabit the branchial chambers of other crustaceans, the form of the female becomes obliquely ovate, the convex side being on the right or the left, according as the animal occupies the right or left side of its host. In some genera (Kepon, Gyge, Ione), a right-hand female has the marsupial plates of the right side overlapping those of the left, while the reverse is the case in a left-hand female. The plates are so imbricated that the last of the five pairs is

outermost. They are attached pair by pair to the first five segments of the peræon as in the Cymothoida. Giard and Bonnier regard them as representing parts of the exopods of the limbs. This seems to require what they admit to be a very hypothetical explanation of an Isopod's leg, namely, that the fourth and fifth joints are the fifth subdivided, and that the small first joint is a fusion of the first and second. By this redistribution the long second joint becomes the third, and thus matches the long third joint so frequently found in the third maxillipeds of the higher Crustacea. But convenient as the hypothesis may be for attaining this piece of symmetry, other grounds for it are not as yet forthcoming. However that may be, the authors show that the structure of these marsupial plates admits of the view that they are to a considerable extent branchial, that is, assist in the oxygenation of the blood. The first pair has a special structure and function. As examined in the genus Cancricepon the larger and upper member of the pair is found to be divided into two portions by a median fold with an outer crest. The front part covers the base of the maxillipeds, the lower part is covered by the opposite plate and floats freely in the marsupial cavity. There are two movements affecting this apparatus, one that alternately lifts and lowers it as a whole, the other alternately lifting and lowering the front and back. By this means a current of water is maintained both to the marsupial plates and to the embryos within them. In the podophthalmous host the water enters at the back and leaves by the front of the branchial cavity, so that in the parasite which lies with its head towards the tail of its host it naturally enters by the upper part and leaves by the lower. In the Entoniscidæ the arrangement is modified to correspond with their position among the viscera.

In regard to the chance of procuring specimens, the authors note that, to obtain the Cryptoniscian stage' and the young female of Athelgue paguri, the pleon of Paguri should be carefully examined in the month of September, and that these interesting forms will be found much less. rare than is often supposed. Dr. Hoek records that on

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examining twelve specimens of Mysis oculata he found six infested with young Dajus mysidis. On the other hand, at the Neapolitan Marine Station, Salvatore lo Bianco had opened about 10,000 Brachyura before he came across a couple of Risso's Ergyne cervicornis, and MM. Giard and Bonnier themselves had to sacrifice tens of thousands to provide the requisite materials for their 'Contributions à l'étude des Bopyriens.'

Should any sensitive persons regret this expenditure of life on a scientific investigation, they must remember that it is perfectly trivial compared with what is continually being exacted for the meaner purpose of tickling man's palate, trivial also compared with the havoc so frequently wrought by storms among creatures of this same class, and further, that against every crustacean destroyed must be set the lives of many others preserved which would else have been its victims.

According to the authors cited only Portunion Kossmanni on Platyonichus latipes can be called common. They opened an enormous number of Porcellana longicornis at ever so many points of the French coast before meeting with a specimen of Entoniscus Mülleri at Concarneau. Both there and at three other places they examined the Pagurid Clibanarius misanthropus without finding a parasite, although at Mahon in Minorca Dr. Fraisse found upon this crustacean a Peltogaster, a Cryptoniscus, an Athelgue, and a Palagyge Thousands of Portunus depurator (Linn.) and Porcellana platycheles had in like manner yielded them no parasites. They feel ready to affirm that our common edible shrimp, Crangon vulgaris, is free from Bopyrids, though they remind us that Crangon munitus, its American congener, is infested by Argeia. The occurrence of Epicaridea may be called, in terms of our present knowledge, very capricious. It not unfrequently happens that, where they occur at all, they occur in some profusion, their hosts suffering as it were from an epidemic attack. One host may undoubtedly carry several species of parasite, but it is an axiom with Giard and Bonnier that both among the Epicaridea and the Rhizocephala no species of parasite has

more than one host. This, if accepted, makes the distinction of species particularly easy. It is, however, a rather wide generalisation. It appears to imply that the larval forms always settle on the same species as that occupied by their parents, or perish unless they do. Yet, if the larvæ of a single brood were dispersed upon hosts of nearly related species, one might expect that to those placed in slightly novel circumstances, some difference of habit would result rather than destruction. Still the important point would remain that in each species of host the parasite is distinct in character, and in favour of their view that it is also distinct in species, Giard and Bonnier urge that 'often among closely related Epicarids there are considerable physiological differences, sometimes even morphological differences relating solely to the male or the embryo, differences too important to be attributed simply to the difference of host.'

In the relations between the Epicaridea and their hosts a very singular circumstance has recently been brought to light. Rathke and other observers had commented on the unexplained peculiarity that the infested prawns and crabs all appeared to be females, and moreover sterile females. De Haan, as heretofore remarked, not unfrequently records sterile females of a different form from the fertile females of the same crab. Giard and Bonnier have shown that an infested female is not always absolutely sterile; but a few years earlier Professor Giard made the extremely interesting discovery that the parasites attack males just as freely as the other sex. Only, under the influence of the invader the distinguishing characters of the male are hindered from development, and a sort of intermediate appearance is permanently retained.

Family 1.-Microniscidae.

This includes the least degraded forms, corresponding in general aspect to the second larval form of the other families. They are parasitic on Copepoda.

Microniscus, Fritz Müller, 1870, is the only genus.