THE BRANCHIAE OF PEN^US. 



267 



As these forniuliB show, those trichobranchiate Crus- 

 tacea, which possess fewer than twenty-one complete 

 branchiae on each side, commonly present traces of the 

 missing ones, either in the shape of epipodites, as in the 

 case of the podobranchire, or of minute rudiments, in the 

 case of the arthrobranchias and the pleurobranchiae. 



In the marine, prawn-like, genus Penaus (fig. 73, 

 Chap. VI.), the gills are curiously modified trichobran- 

 chise. The number of functional branchiae is, as in the 

 lobster, twenty ; but the study of their disposition shows 

 that the total is made up in a very different way. 



The branchial formula of Penceus. 



+ 6 ep. 



7 = 20 + G ep. 



This case is very interesting; for it shows that the 

 whole of the podobrancliiae may lose theii" branchial charac- 

 ter, and be reduced to epipodites, as is the case with the 

 first in the crayfish and lobster, and indeed in most of 

 the forms under consideration. And since all but one of 

 the somites bear both arthrobranchiae and pleurobranchiae, 



