1878.] 



AND DISTRIBUTION OF THJC CRAYFISHES. 



770 



vessels, and take the place ot 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 

 phyllobranchiae in their small number and in their disposition, in- 

 asmuch as they are directed obliquely to the stem and not at right 

 angles to it. Nevertheless it is interesting to find, in both Axlus 

 and Thalassina, a certain approach to the phyllobranchiate type, 

 which is completely reached in Qebia and Callianassa. 



The Branchial formula of Thalassina. 



Somites -r, , Arthrobranchise. th 



„ t ,, ■ Podo- Pleuro- 



and their , , . ,. » -. , , . 



„ A branchiae. . , . t> , • branchuv. 



appendages. Anterior. Posterior. 



VII = 



VIII (ep) 1 1 = 



IX 1 1 1 () = 



► X 1 1 1 = 



XI 1 1 1 = 



XII (ep) 1 1 = 



XIII 0(ep) 1 1 = 



XIV = 







2 +ep 



3 



3 



3 



2 +ep 



2 +ep 







3 + 3ep +6 + 



+ = 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 podobranchiaa 

 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 

 maxillipeue, 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 P do- 



and their , i • 



j branchiae, 

 appendages. 



VII (ep) 



VIII O(epr) 



IX 0(ep) 



X 0(ep) 



XI (ep) 



XII (ep) 



XIII (ep) 



XIV 



Arthrobranchise. 



Anterior. 



? 



1 

 1 

 1 

 1 

 1 

 1 

 D 



Posterior. 

 



1 

 1 

 1 

 1 

 1 

 1 







Plenro- 

 branchise. 



= 



1 = 

 1 = 

 1 = 

 1 = 

 1 = 

 1 = 

 1 = 



(ep) 

 3 + ep r 

 3-t-ep 

 3 + ep 

 3 + ep 

 3 + ep 

 3 + ep 

 1 



+ 7 ep +r + 6 + 



+ 



= 19 + 7ep + r 



