1878.] 



AND DISTRIBUTION- OF THE CRAYFISHES. 



785 



the thirteenth-and four pleurobranchi*, one for each of the four 

 posterior thoracic somites. 



The Branchial formula of Dromia. 

 Arthrobranchiae. 



Somites 



and their . Pod ,°: 



appendages. Manchias. 



VII (ep) 



VIII 1 



IX 1 



X 



XI o 



XII o 



XIII o 



XIV o 



2 + ep. + 5 + 



+ 4 = 



0(ep) 



1 



3 



2 



3 



3 



3 



1 



16+ ep. 



On comparing this branchial formula with that of Homarus the 

 relation between the two is obvious. In fact, if the tbre posterior 

 podobranchw of the Lobster are suppressed, and the nex i/reduced 

 to an epipod.te, the branchial formula becomes the same as that of 

 Drom la and th remaining differences between the respiratory o ™ 

 of the two result from the modification in form and structure ofthe 

 b anchial elements which remain. Thus it is a permissible, f not a 

 probable suggestion that, ust as the Anomomorpha may have been 

 derived from the modification of some form allied to ^ald the 

 Caridomorpha from some form allied to Stenopu* znTLvus so 

 the Carcmomorpha (= the Brachyura and the Raninidi with 

 Homola and Dromia) may have proceeded from some Homarine 



However this may be, the actual morphological relations of the 

 oytrfSng^^ 



CRUSTACEA. 



THORACOSTRACA OR PODOPHTHALMIA. 



III. PHYLLOBRANCHIATA. 

 Caridomorpha. Anomomorpha. Carcinomorpha. 



II. TBICHOBRANCHIATA. 



r^neii^ StenopU*. TkalaeHmL. Axiid*. Bom arid«. PotamoUida. 



Pali nurida . 

 Parastacidtt. 



Euphait4td&. 



Mysida 



I. ABRANCHIATA. 



Stvlophora. 



ASTVLA. 



