OCR Text |
Show 1878.] AND DISTRIBUTION OF THE CRAYFISHES. 775 copsis (fig. 7, v). The hinder and upper seta? of the bases of the podobranchiae are similarly hooked ; but, as in the other species, the anterior setae are straight, or only slightly curved at the extremities. The Branchial formula of Astacoides madagascariensis. Somites p , Arthrobranchia?. „, and their * °c10- ^ Pleuro-appendages. bra™hia?. ^ ^ Fosteri^ branchiae. VII 0 (ep r) 0 0 0 = (ep r) VIII 1 r 0 0 = 1 + IX I 1 0 0 = 2 X 1 1 r 0 = 2 + XI 1 1 r 0 = 2 + XII 1 1 r 0 = 2 + XIII 1 1 r 0 = 2 + XIV 0 0 0 1 = 1 r r r r 6 + epr + 5 + r + 4r + 1 = 12 + epr+5r In Astacoides, therefore, the branchiae have suffered more reduction than in any other known Crayfish ; aud this reduction is, as it were, a continuation of the process already commenced in Engeeus and Paranephrops, in which the anterior pleurobranchiae and the posterior arthrobranchiae are small, or even rudimentary. III. THE CLASSIFICATION OF THE CRAYFISHES. Whatever may be the variation in the structure of the branchiae of the different species of Crayfish, it will be observed that they all agree in possessing podobranchia?, or branchia? attached to the coxopodites, of the six middle thoracic appendages, and that these are either not at all, or incompletely, differentiated into a branchial and an epipoditic division. Moreover Astacopsis, Chceraps, Engeeus, Paranephrops, Parastacus and Astacoides, in which the apices of the podobranchiae are not separated into a branchial plume and a well developed lamina, present a less-differentiated type of branchial structure than that which obtains in Astacus and Cambarus. Thus the structure of the branchia? in the Crayfishes separates them into two groups, of which I propose to term the latter the P O T A M O B I I D J B , and the former the PARASTACID^E. In the P A R A S T A C I D ^ ; the podobranchia? are devoid of more than a rudiment of a lamina, though the stem may be alate. The podobranchia of the first maxillipede has the form of an epipodite ; but, in almost all cases, it bears a certain number of well-developed branchial filaments. The first abdominal somite possesses no appendage in either sex; and the appendages of the four following somites are large. The telson is never completely divided by a transverse suture. More or fewer of the branchial filaments of the podobranchia? are terminated by short hooked spines ; and the coxopoditic setae, as well 51* |