CLASS OF CRUSTACEA. 



439 



arch of the flancs (Figs. 426 and 428), and are enclosed in 

 two large cavities situated on the sides of the thorax, and 

 comprised between the carapace and the arch of which we 

 have just spoken, an arrangement which does not occur in any 

 other animal of this class. The respiratory cavity communi- 

 cates externally by two openings; the one serving for the 

 entrance of the water, almost always situated between the 

 base of the limbs and the edge of the carapace (r, Fig. 425), 

 the other destined for the escape of this liquid is placed at the 

 sides of the mouth. Finally, the renewal of the water on 

 the surface of the branchiae is caused by the movements of a 



Fig. 428. Respiratory Apparatus of a Palemon.* 



large valvule situated near this latter opening, and formed by 

 a lamellated appendix of the jaws of the second pair (c. Fig. 

 123 ; i, Fig. 428). In other Crustacea, the squilli (shrimps) 

 for example (Fig. 429), the branchiae have the form of 

 bunches of feathers, and in place of being enclosed within the 

 thorax, float freely, externally, and are attached to the abdo- 

 minal limbs ; in others, still, as in the crevettes des ruisseaux 

 (prawns of rivulets) and the talytri, they are formed of mem- 



* a, rostrum ; b, carapace ; c, base of the antennae ; d, base of the abdomen; 

 e, base of the limbs ; f, branchiae ; g, dotted line, pointing out the inferior edge 

 of that portion of the carapace which covers the branchiae, and which has 

 been removed in this preparation ; h, efferent canal of the respiration ; i, 

 valvule ; j, extremity of the efferent, or expiratory canal. 



