202 



THE CRUSTACEA 



but which in other cases undergo many modifications for prehensile, 

 natatory, and other functions. The coxopodite has the form of a 

 complete segment movably articulated with the body only in the 



Asellota (Fig. 122, ex). In all 

 other Isopoda it is more or less 

 completely fused with the body 

 __ | and is often expanded into a coxal 

 plate overhanging the attachment 

 of the limb and replacing the 

 pleural expansion of the somite 



Thoracic somite of Jaera marina, separated to which it belongs. An interest- 



^cTeS'Tco^me^^K^t": - ing series illustrating this substitu- 



tion can be traced within the 



family Idoteidae. In Idotea hectica (Fig. 123, A), the pleural plates 

 (pi) of the thoracic somites are well developed and completely 

 cover the coxopodites (c.r), of which the outline can be traced on the 



FIG. 122. 



B 



C. 



FIG. 123. 



Margin of thoracic somites of, A, Iiloten hectica ; B, 7. ochotensts; C, Chiridotea sahini. The 

 tipper figures show the under surface, the lower the upper surface, ex, coxai plate (dotted); 

 pi, pleural plate ; o, rudiment of oostegite. 



under-side of the plates near the outer edge. In other species of 

 the same genus (Fig. 123, B) the coxal plate projects beyond the 

 outer margin of the pleuron for a part of its length so as to be 

 visible on the upper surface. In yet other species of the genus 

 and in the allied Chiridotea (Fig. 123, C), the pleura are no longer 

 to be distinguished, and their place is taken by the greatly de- 

 veloped coxal plates, which are marked off, on the dorsal surface, 

 by distinct sutures, generally allowing a slight amount of move- 



