1878.] AND DISTRIBUTION OF THE CRAYFISHES. 77ft 



copsis (fig. 7, v). The hinder and upper seta; of the hases of the 

 podobranchise are similarly hooked ; but, as in the other species, 

 the anterior setae are straight, or only slightly curved at the ex- 

 tremities. 



The Branchial formula of Astacoides madagascarie 



nsis. 



Somites p Arthrobrancbise. 



and their . yod P: , *. Pleuro- 



nppendages. branchl «- Anterior. Posterior. h ™™ hi *- 



J T \\ O(epr) 0= (ep r) 



IX 1 1 0=2 



Jr J J r 0=2 + 



X1 1 r 0=2+ r 



XII I 1 1 r 0=2+ r 



XIV I = I 



6 + epr + 5 +r + 4 r + 1 = 12+epr+5r 



In Astacoides, therefore, the branchiae have suffered more reduc- 

 tion than in any other known Crayfish ; and this reduction is, as it 

 were, a continuation of the process already commenced in Engeeus 

 and Paranephrops, in which the anterior pleurobranehiae and the 

 posterior arthrobranchiae are small, or even rudimentary. 



II T. The Classification of the Crayfishes. 



Whatever may be the variation in the structure of the branchise of 

 the different species of Crayfish, it will be observed that they all agree 

 in possessing podobranchiae, or branchiae attached to the coxo- 

 podites, 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, (Jhceraps, 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 Gambarus. 



Thus the structure of the branchiaa in the Crayfishes separates 

 them into two groups, of which I propose to term the latter the 

 Potamobiid^e, and the former the Parastacid^e. 



In the Parastacid^e the podobranchise are devoid of more than 

 a rudiment of a lamina, though the stem may be alate. The podo- 

 branchia 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 podobranchiee are 

 terminated by short hooked spines ; and the coxopoditic setae, as well 



51* 



