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Show 782 PROF. T. H. HUXLEY ON THE CLASSIFICATION [June 4, If the branchial filaments of Axius were shortened and widened, the structure of the branchiae would approach that which obtains in Gebia and Ccdlianassa, which are truly phyllobranchiate. But in other respects there is a wide interval between these genera, on the one hand, and Axius and Thalassina, which are ordinarily associated with them among the Thalassinida?, on the other hand; for the podobranchia? have entirely disappeared on the six hinder thoracic limbs, and even on the first or second maxillipede they are represented only by rudimentary epipodites. There are no pleurobranchiae; and the total number of gills is reduced to five pairs of arthrobranchiae on each side. The Branchial formula of Gebia and Callianassa. Somites and their appendages. VII VIII IX. X XL . XII. XIII... XIV, Potjo-branchia?. . 0 0 . 0 0 . 0 . 0 . 0 . 0 0 + Arthrobranchia?. Anterior. Posterior. 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0 5 + 5 Pleurobranchia?. 0 0 0 0 0 0 0 0 + o = = = = = = = = -- 0 0 2 2 2 2 2 0 10 In the almost complete abortion of the podobranchia?, and in the presence of ten arthrobranchia? attached in pairs to the middle thoracic somites, Gebia and Callianassa agree with Porcellana, Galathea, Lithodes, Pagurus, and Bemipes. But in Galathea and Porcellana the four hindermost pleurobranchia? are present, making fourteen gills on each side; in Lithodes and Pagurus the penultimate pleurobranchia exists, making eleven ; in Bemipes there is no pleurobranchia, and only nine arthrobranchia?, viz. one on the ninth and two for each of the four following somites, are present. In this group, which nearly corresponds with the Anomala of De Haan, and which I propose to term the " Anomomorpha," there is every degree of modification-from such typically Macrurous forms as Gebia and Galathea to such pseudo-carcinoids as Lithodes and Porcellana. It is interesting to remark, however, that, while in Thenus and Ibacus the process of modification has chiefly affected the head, in the Anomomorpha the characteristic changes are more marked in the abdomen. In none of the latter are the basal joints of the antenna? fixed, nor are distinct orbits formed. It is easy to understand the possibility of the derivation of the Anomomorpha from some form allied to Axius and Thalassina (but with four pleurobranchia?) by the further reduction, and final almost complete disappearance, of the podobranchia?, while the biserial filaments of the other gills flattened out and became lamellar. The Prawns and Shrimps ("Salicoques" of Milne-Edwards, Cannes |