THE BRANCHI^ OF PEN^US. 



267 



As these formulae 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 podobranchiae, or of minute rudiments, in the 

 case of the arthrobranchiae and the pleurobranchi^. 



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

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

 chiae. The number of functional branchi^ 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 Penmis. 



Pleuro- 

 branchioe. 



= 



+ ep. 

 + ep. 

 + ep. 

 + ep. 

 + ep. 

 + ep. 



+ 6 ep. + 7 + 



20 + 6 ep. 



This case is very interesting; for it shows that the 

 whole of the podobranchise may lose their 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 aU but one of 

 the somites bear both arthrobranchi^ and pleurobranchi^, 



