| OCR Text |
Show 766 PROF. T- H. HUXLEY ON THE CLASSIFICATION [June 4, the last thoracic somite. There is also an anterior series of six arthrobranchiae, and a posterior series of five (p. 760, fig. 4, arb), disposed as in Astacus. But instead of the two rudimentary pleuro-branchiae borne by the twelfth and thirteenth somites, in Astacus, there are three perfect branchiae attached to the eleventh, twelfth, and thirteenth somites (fig. 4, n-\zplb), in positions which, in the case of the two latter, exactly correspond with those of the rudimentary pleurobranchiae of Astacus. The pleurobranchiae and the arthrobranchiae are very similar to those oi Astacus ; but the inner surface of the stem is channelled by a sort of groove, the edges of which are pretty sharp. I could find no hooks, nor spine3, nor hooked papillae on any part of these branchiae. The podobranchise (fig. 5, i, II, in) resemble those of Astacus in their general form; but the basal portion is relatively larger and the stem stouter, while, at first sight, the lamina appears to be altogether wanting. Close examination, however, shows that the apex of the branchia is distinguishable into an apical plume and a lamellar appendage, but that the latter is relatively very small; in fact, it is a mere outgrowth of the inner wall of the stem, and is neither bilobed nor plaited. Moreover its surface is beset with numerous filamentous prolongations, which are altogether similar to ordinary branchial filaments, except that the extremity of each, instead of being smooth and rounded, is provided with a short recurved hook-like seta (fig. 5, n a, iv). The groove on the inner or posterior face of the stem is broader than in Astacus. It is bounded on the inner side by a decurrent fringe, which runs down onto the base and stops there, as in the foregoing genera (fig. 5, n). This fringe is provided with long slender curved setae, each beset with imbricated scales and terminated by a recurved book (fig. 5, v). The external ridge is wider than in Astacus, and is provided with similar hooked setae. It may be said, in fact, that the whole lower half of the posterior margin of the stem in Astacoqisis has the character of the posterior margin of the base, while in Astacus there is a sharp demarcation between the base and the stem. In principle, therefore, the podobranchiae oi Astacopsis are similar to those of Astacus ; and the main difference between the two is that the lamina in the former is represented by a slight expansion of the stem, which is neither bilobed nor plaited, while its surface is covered with cylindrical filaments terminated by hooked spines. In Astacus, as in Cambarus, on the other hand, the lamina is large, bilobed, plaited, and the place of the filaments is taken by mere papillae terminated by similar hooks. Moreover, in Astacus and Cambarus the setae of the base are not hooked. The appendage of the first maxillipede is similar to that of Astacus in form; but on the outer surface of the outer lobe there are sixteen or eighteen short branchial papillae, some of which, but apparently not all, are terminated by hooks ; in fact, except in size, they quite resemble the filaments of the other branchiae. In this Crayfish, therefore, the first podobranchia is not reduced |
