THE BRANCHIAE OF PENjEUS. 



267 



As these formulae show, those trichobranchiate Crus- 

 tacea, which possess fewer than twenty-one complete 

 branchiae on each side, co mmonly present traces of the 

 m issing one s, either in the shape of epipodites, as in the 

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

 case of the arthrobranchise and the pleurobranchife. 



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

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

 chiae. 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 + 6 ep. 



This case is. very interesting; for it shows that the 

 whole of the podobranchiae maylose their branchial charac- 

 terj_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, 



