6

type by having eyes, which are small but distinct, the second antennæ shorter, the second joint in the limbs of the person more dilated, and the pleon furry. In describing this species Mr. Chilton mentions that the third pleopods have on the outer margin of the basal portion an ovate appendage which perhaps represents the epipodite.' He finds the males distinguished from the females not only by the special organs at the base of the seventh pair of limbs, but by rather stronger hands in the first pair, and a clasping arrangement in the last two joints of the fourth pair. The female appears to have brood-plates on the second, third, and fourth segments of the peræon.

The figures in Plate XVI. are copied from the papers in which Mr. Chilton describes the two notable species under discussion.

While the mouth-organs and pleopods make Phreatoicus an unquestionable Isopod genus, the affinity with the Amphipoda appears not a little remarkable. The lateral compression of the body, the downward lateral production of the pleon-segments, the arrangement of the limbs of the peræon, the considerable development of the pleon, the two-jointed branch in several of the pleopods, the strong indication of a distinction between the sixth pleonsegment and the telson, and above all the position and structure of the uropods, form a group of characters which bring this genus nearer to the Amphipoda than any other Isopod can claim to be. In the limbs of the peræon the third joint is not very short, as it usually but by no means always is in the Gammaridea, nor does the fourth joint in the first pair underride the fifth as is usual in the gnathopods of the Gammaridea. On the other hand the clasping arrangement of the fourth pair of limbs is strikingly parallel to what is found in the Orchestiidæ, and, as Mr. Chilton points out, the expansion of the second joint of the limbs in Phreatoicus australis is similar to what is found in so many of the Amphipoda. In his admirable discussion of all the characters Mr. Chilton is inclined to disallow the Phreatoicidæ any special proximity to the Amphipoda, and is thus induced to remark that

APPROXIMATE TO THE AMPHIPODA

391

the uropoda 'present no greater resemblance to the Amphipoda than to several of the Isopoda such as Asellus,' although the position alone would suffice to distinguish them from those of Asellus, and if disjoined from the animal there is absolutely nothing about them that would enable any one to say that they belonged to any Crustacean except their own or some Amphipod genus. The spine at the lower end of the peduncle occurs as so often in the Amphipoda. The genus and its two species are beyond dispute remarkable, requiring to be placed in a separate family, and though some may prefer to class this among the Asellota, I have ventured to think that a separate tribe Phreatoicidea should be instituted to receive it.

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. Hcek records that on