314 THE ANATOMY OF INVERTEBRATED ANIMALS. 



waters, but the great majority are marine ; and among them 

 are many peculiarly modified parasitic forms (Fig. 82, Cymo- 

 thoa Bopyrus). The composition of the head and mouth 



FIG. 82. Cymothoa. The letters and figures have the same signification as in Fig. 

 81, except Ab, abdominal appendages in Fig. A. 



in the Isopoda is essentially the same as in the Amphipoda, 

 though differing considerably in details. The branchiae of 

 the thoracic members are absent, their functions being per- 

 formed by the endopodites of some of the abdominal mem- 

 bers, which are soft and vascular. The three anterior pairs 

 of thoracic members are usually directed forward the four 

 posterior pairs backward. In some Isopoda the abdominal 

 somites, partly or wholly, coalesce with one another. 



In all the Edriophthalmia the alimentary canal is straight 

 and simple, and its anterior gastric dilatation, frequently 

 strongly armed, is situated in the head. The liver is repre- 

 sented by a variable number of straight caeca. 



Occasionally there are one or two caeca which open into 



