CLASS OF CRUSTACEA. 



477 



maining adapted for this kind of locomotion, they become 

 suited to act as so many small spades wherewith to dig the 

 ei'rth, and in that case they are enlarged and lamellated 

 towards the extremity (Fig. 508) ; and still, finally, in others, 

 they terminate in forceps, and become instruments of prehen- 

 sion, fulfilling at the same time their ordinary functions of 

 instruments of locomotion (Fig. 143). In the swimming 

 Crustacea, such as the craw-fish and the lobster (Fig. 504), 

 the palsemon (Fig. 507), &c., the abdomen presents in general 

 a considerable 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 very 

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

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

 constitutes then a moveable apron placed on the lower surface 

 of the body between the limbs. 



Fig. 509. Nervous System of a Crab : the Maia; Sea Spider.* 



564. The nervous system is composed of a double series 

 of ganglions, situated on the ventral aspect of the body near 

 the median line. Their number corresponds in general to 



* Carapace, laid open : a, exterior antennae ; ;/, eyes ; e, stomach ; c, 

 brain ; no, optic nerves ; co, oasophagal collar ; ms, stomato-gastric nerves ; 

 t, thoracic gangliouary mass ; np, nerves of the limbs ; no., abdominal nerve. 



