18/8.] 



AND DISTRIBUTION OF THE CRAYFISHES. 



781 



Somites 



and their 



appendages 



VII... 



VIII... 



IX... 



X... 



XI... 



XII.. . 



XIII... 



XIV. . . 



The Branchial formula of Penseus brasiliensis. 



Arthrobranchise. 



Podo- 

 branchias. 



(ep) 

 0(ep) 

 0.(ep) 



0(ep) 



0(ep) 



0(ep) 











Anterior. Posterior. 



1 (small) 



1 1 



1 1 



] 1 



1 1 



1 1 



1 1 







Pleuro- 

 branchia;. 



= 



1+ep 

 3 + ep 

 3 + ep 

 3 + ep 

 3 + ep 

 3 + ep 

 3 

 1 



+ 6ep+7 + 6 + 7 = 20 + 6 ep. 



The arthrobranchia of VII. is very small ; in fact, it is a mere 

 tuft of dichotomously divided branchial filaments. 



The results of this examination of the principal forms of those 

 Trichobranchiata which possess more than one kind of branchiae rnav 

 be thrown into a tabular form, as follows : — 



I. No appendages to the first abdominal somite in either 



sex. Astyla. 



1. Podobranchia3 undivided Parastacid^e. 



2 Podobranchias divided into branchial and epipoditic 



portions Palinuridj;. 



II. Appendages of the first abdominal somite almost always 



present, and specially modified in the males. Stylojjhora, 



1. Podobranchite partially divided Potamobiid.e. 



2. Podobranchia; completely divided or reduced to epi- 



podites. 



a. Pleurobranehiae not more than four. 



a. Posterior pleurobranchia present, and the 

 most posterior of the podobi-anfliia 1 com- 

 plete HoMARlD.B. 



Homarus, Nephro2)s. 

 /3. Posterior pleurobranchia wanting, and more 

 o? fewer of the posterior podobranchia 

 reduced to epipodites. 



a. Branchiae with only filamentous pro- 



cesses Axius. 



b. Branchiae with foliaceous as well as 



filamentous processes Thalassina. 



b. Pleurobranchia? more than four. 



a. Branehias filamentous Stenopus. 



j3. Branchiae ramose Penaus. 



All the Trichobranchiata are Macrura, in the sense of having the 

 abdomen and its appendages well developed ; but, in Ibacus and 

 Thenus, the abdomen becomes shortened and the cephalothorax wide, 

 while the antennary and the ocular regions are modified in a manner 

 very similar to that which gives rise to the peculiar "face" or 

 "metope" of the typical Brachyura. A very little further modifi- 

 cation would convert Thenus, for example, into a trichobranchiate 

 Crab. Such forms as these, which simulate the Brachyura, and yet 

 differ profoundly from them, may be termed " pseudo-carcinoids." 



