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Show 776 PROF. T. H. HUXLEY ON THE CLASSIFICATION [June 4, as those which beset the stems of the podobranchia?, have hooked apices. In the P O T A M O B I I D ^ ; the podobranchia? of the second, third, fourth, fifth, and sixth thoracic appendages are always provided with a large plaited lamina. The podobranchia of the first maxillipede is converted into an epipodite, entirely devoid of branchial filaments. The first abdominal somite invariably bears appendages in the male, and usually in both sexes. In the male these appendages are styliform, and those of the second somite are always peculiarly modified. The appendages of the four following somites are relatively small. The telson is usually completely divided by a transverse suture. None of the branchial filaments are terminated by hooks ; nor are any of the coxopoditic seta? or the longer setae of the podobranchiae hooked, though hooked tubercles occur on the stem and on the lamina of the latter. The coxopoditic seta? are always long and tortuous. It is worthy of notice that the Parastacida? agree with the Palinurida? and Scyllaridee in the abortion of the appendages of the first abdominal somite, and in possessing hooked setae, while in the Potamobiidae, as in the Lobsters, the seta? are not hooked, and, as in almost all the Podophthalmia, except the Palinuridae and Scyllarida?1, the appendages of the first abdominal somite are present, and are specially modified in the males. Of the six genera of the Parastacida?, Astacoides is widely separated from all the rest by the reduction of the number of its functional branchia? to twelve, while all the other genera at present known have, at fewest, twenty, and usually twenty-one, branchiae. Astacopsis, Chceraps, Engceus, and Parastacus have, in common, a long epistoma, the surface of which is flattened, the basal joint of the antenna? fixed by the overlapping edge of the cephaloste-gite, the posterior thoracic sterna narrow, the coxopodites of the hinder thoracic limbs large and approximated in the middle line. The rostrum and the antennary squame are short. Engceus is distinguished among these, not only by the narrowness of the first abdominal somite, but by the form of the anterior part of the head, with its short and deflexed rostrum, and very small antennary squame. In Chceraps the podobranchia? are alate, in Astacopsis they are not. Parastacus somewhat resembles Chceraps in its alate podobranchia?, but differs from all the rest by possessing long and tortuous coxopoditic seta?. I have nothing to add to the distinctive characters of the two genera of the Potamobiidae, Astacus and Cambarus, already given by Erichson, Hagen, and others. All the branchia? of the Crayfishes consist of a stem beset with numerous cylindrical filaments. In fact, they are typical examples of what are termed by Milne-Edwards " branchies en brosse," and may be called " trichobranchia?," in contradistinction to the lamellar 1 In Gebia, Calliaxis, and Porcellana, the first abdominal appendages are rudimentary or abortive in the male sex. |
