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Show 1878.] AND DISTRIBUTION OF THE CRAYFISHES. 785 the thirteenth-and four pleurobranchia?, one for each of the four posterior thoracic somites. Somites and their appendages. VII vm IX X XI XII XIII XIV The Branchial formula of Dromia PodoArthrobranchia?. branchia?. 0(ep) 1 1 0 0 0 0 0 Anterior. 0 0 0 Posterior. 0 0 1 1 1 1 1 0 Pleurobranchia?. 0 = 0 = 0 = 0 = 1 = 1 = 1 = 1 = 0(ep) 1 3 O Ad 3 3 3 1 2 + ep. + 5 + + 4 = 16+ ep. On comparing this branchial formula with that of Homarus, the relation between the two is obvious. In fact, if the three posterior podobranchia? of the Lobster are suppressed, aud the next is reduced to an epipodite, the branchial formula becomes the same as that of Dromia, and the remaining differences between the respiratory organs of the two result from the modification in form and structure of the branchial elements which remain. Thus it is a permissible, if not a probable, suggestion that, just as the Anomomorpha may have been derived from the modification of some form allied to Axius, and the Caridomorpha from some form allied to Stenopus and Peneeus, so the Carcinomorpha (= the Brachyura and the Raninidae, with Homola and Dromia) may have proceeded from some Homarine stock. However this may be, the actual morphological relations of the Thoracostraca appear to me to be represented with tolerable accuracy by the following scheme :- CRUSTACEA. THORACOSTRACA OR PODOPHTHALMIA. III. PHYLLOBBANCHIATA. CARIDOMORPHA. ANOMOMORPHA. CARCINOMORPHA. II. TRICHOBRANCHIATA. Palinurida. Pen&idce. ' StetwpidcE. Thalassinidts. Axiidce. Homaridce. Potamobiidae. Parastacida. MuphaitAida, Sfysidce. I. ABRANCHIATA. STYLOPHORA. ASTYLA. |