ZOOLOGY. 



aceous, membranous, and exclusively adapted for swimming 

 (Fig. 432); in others they have the form of small flexed (like 

 elbows) columns, articulated, and disposed only for walking; 

 in others still, besides remaining adapted for this kind of 

 locomotion, they become suited to act as so many small 

 spades wherewith to dig the earth, and in that case they are 

 enlarged and laraellated towards the extremity (Fig. 1-22); 

 and still, finally, in others, they terminate in forceps, and 

 become instruments of prehension, fulfilling at the same 

 time their ordinary functions of instruments of locomotion 

 (Fig. 122). In the swimming Crustacea, such as the craw- 



Fig. 423. Nervous System of a Crab : the Maia.* 



iish and lobster, the langouste (Fig. 418), the palsemons 

 (Fig. 421), &c., the abdomen presents in general a con- 

 siderable development, and terminates by a large fin, so as to 

 become the principal organ of locomotion; but in those 

 intended to walk more than they swim, it is in general vcrv 

 small, and folded under the thorax : in the crabs, for example, 

 this portion of the body is reduced almost to nothing, and 



* Carapace, laid open : a, exterior antenna?; y, eyes ; e, stomach; c, 

 l.rain ; no, optic nerves ; co, (Bsophageal collar; tu, stomato-eastric nervea; 

 /, thoracic ganglionary mass ; *p t nerves of the limbs ; ma, abdominal nerve. 



