BRANCHIAL FORMULAE. 



265 



This tabular " branchial formula " exhibits at a glance 

 not only the total number of branchiae, but that of each 

 kind of branchia ; and that of all kinds connected with 

 each somite ; and it further indicates that the podo- 

 branchia of the first thoracic somite has become so far 

 modified, that it is represented only by an epipodite, with 

 branchial filaments scattered upon its surface. 



In Paliuurus, these branchial filaments are absent and 

 the branchial formula therefore becomes 



6 + ep. + 6 



+ 4 = 21 + ep. 



In the lobster, the solitary arthrobranchia of the eighth 

 somite disappears, and the branchiae are reduced to twenty 

 on each side. 



In Astacus, this branchia remains ; but, in the English 

 crayfish, the most anterior of the pleurobranchiae has 

 vanished and mere rudiments of the two next remain. 

 It has been mentioned that other Astaci present a 

 rudiment of the first pleurobranchia. 



