236 



NERVOUS SYSTEM OP CRUSTACEA. 



locomotion ; but in those which are destined to walk rather than 

 to swim, this part is generally very small and bent under the 

 thorax. In the Crabs, for instance, this portion of the body is 

 reduced to almost nothing, and forms a sort of moveable append- 

 age, which is seen on the inferior surface of the body beneath 

 the legs (Fig. 458). 



774. The Nervous System of Crustacea consists of a double 

 series of ganglia, situated on the ventral surface of the body, 

 near the central line. In general their number corresponds 

 to that of the distinct segments composing the body ; and the 

 first pair is always 

 placed on the head, in 

 front of the oesophagus, 

 where it forms a sort of 

 brain. The arrange- 

 ment of the thoracic 

 and abdominal ganglia, 

 however, varies con- 

 siderably; sometimes 

 they are placed at equal 

 intervals, and form, 

 with their cords of 

 communication, a chain 

 extending from one nd 

 of the body to the 

 other ; sometimes they 

 approach one another 

 more or less closely ; 

 and sometimes they 

 are united, forming a 



single mass, situated about the middle of the thorax (Fig. 461, t). 

 It may be remarked that this concentration of the nervous 

 system becomes more and more complete, as the animal acquires 

 a higher organisation. The Crustacea have in general but very 

 limited faculties ; and none among them present much to interest, 

 as to their habits. The eyes are formed nearly in the same manner 

 as in Insects ; sometimes they are simple, but in general they 

 are compound ; and in all the most perfect Crustacea, these 



FIG. 461.' NERVOUS SYSTEM OF CRAB (Maia) : ca, 

 upper part of the shell laid open ; a, antennae ; 

 y, eyes ; e, stomach ; c, cephalic ganglion ; no, optic 

 nerves ; co, oesophageal collar ; ns, stomato-gastric 

 nerves ; t, thoracic ganglionic mass ; np, nerves of 

 the legs ; na, abdominal nerve. 



