1332 





CRUSTACEA. 



articulation, and serve to retract the leg to the shell. These muscles 

 probably co-operate with the posterior in the depression of the leg. 



If these oar-like legs struck the water with the same broad ex- 

 panded surface, in their backward motion, as in their forward pro- 

 pelling action, the animal would advance but slowly, if at all, as the 

 latter would be counteracted by the former. There is a provision 

 against such a defect, in the muscles moving the several joints of 

 these legs, by the action of which, the terminal portions receive a 

 partial revolution, and cut the water, when drawn backward, by their 

 thin anterior edge. Their special adaptation for this purpose is appa- 

 rent, even in the pinnula terminating the leg, which instead of being 

 ciliated on both edges, is furnished anteriorly with a thin membranous 

 expansion. 



These legs appear to be the only organs for walking as well as 

 swimming. 



The principal extensors of the third pair of natatories, or the aj^ron, 

 are four in number; two (y, z) arise on the back near the medial line, 

 and pass laterally to the outer insertion of the apron. One of the 

 remaining two (x) arises just above the posterior sinus, and the other 

 from the inner margin of this sinus; both are attached on the back, 

 and inserted near the articulation of the sternum. The flexor muscles 

 arise below, just outside the apron, and occupy the greater part of its 

 interior. A single muscle is attached near the articulation of the 

 sternum, and passes into the basal portion. 



This apron, appended to the cephalothoracic segment, forms the 

 anterior portion of the body into a large, broad cup, which is perfectly 

 closed, with the exception of a small opening at each of the posterior 

 sinuses. These are provided with a folded membrane, furnished 

 with muscles capable of drawing it over and completely shutting 

 the opening. The membranous margin of the animal near the an- 

 tennae, has also a fold by which a small leak, if it be such, is closed. 

 Considering these several provisions, it is probable, that the whole of 

 this anterior portion of the animal is especially adapted to enable it 

 to attach itself firmly during the rapid motions of the fish, and that 

 the small marginal cups in front are relied on, only while the fish is 

 stationary, or but slowly moving. 



The remaining pair of legs are moved by short slender muscles, and 

 seem to possess little power. They usually hang loose and motionless 



