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Show 1878.] AND DISTRIBUTION OF THE CRAYFISHES. 779 vessels, and take the place of the branchial filaments, which are sometimes very few, and confined altogether to the basal region of the branchia. These branchial plates differ from those of the true phyllobranchia? in their small number and in their disposition, inasmuch as they are directed obliquely to the stem and not at right angles to it. Nevertheless it is interesting to find, in both Axius and Thalassina, a certain approach to the phyllobranchiate type, which is completely reached in Gebia and Callianassa. The Branchial formula of Thalassina anTtneir Pod°- Arth™^anchia, neuro-appendages. branchl*- AnterioT" Posterior. branchia!* VII 0 0 0 0 : VIII 0(ep) 1 1 0 IX 1 1 1 0 X 1 1 l 0 XI 1 1 1 0 XII 0 (ep) 1 1 0 XIII 0(ep) I 1 0 XIV 0 0 0 0 0 3 3 3 2 2 0 = 2 +ep + ep + ep 3+3ep +6 + + 0 = 15 + 3 ep. In Stenopus hispidus the branchiae resemble those of Astacopsis in structure, the filaments being loose, slender, and multiserial. But the disappearance of the branchial plumes from the podobranchia? has gone still further than in Thalassina; and all these organs are reduced to delicate, almost vesicular epipodites entirely devoid of any proper branchial filaments, except in the case of the second maxillipede, in which a few such filaments are attached to the base of the epipodite. Moreover the number of the pleurobranchiae is increased to seven. The Branchial formula of Stenopus. Somites Podo. and their branchiffi. appendages. VII 0 (ep) VIII 0(epr) IX 0(ep) X 0(ep) XI 0 (ep) XII 0 (ep) XIII 0 (ep) XIV 0 Arthrobranchia?. Anterior. ? 1 1 1 I 1 1 0 Posterior. 0 1 1 1 1 1 1 0 Pleurobranchia?. 0 = 1 = 1 = 1 = 1 = 1 = 1 = 1 = (ep) 3 + ep r 3 + ep 3 + ep 3 + ep 3 + ep 3 + ep 1 0 + 7ep+>* + 6 + 6 + 7 = 19 + 7ep + r |