488 



THE SENSORIAL lUNCTlONS. 



sea shore, and which is called the Taiitrus locusta, 

 438 ^ or Sand-hopper, (Fig. 438). The 



central parts of its nervous system 

 are seen in Fig. 439, which repre- 

 sents the abdominal side of this animal laid open, 

 and magnified to twice the natural size. The two 

 primary nervous cords, which run in a longitu- 

 dinal direction, are here perfectly distinct from one 

 another, and even separated by a small interval : 

 they present a series of ganglia, which are nearly 

 of equal size, and equidistant from one another ; 

 one pair corresponding to each segment of the 

 body,* and united by transverse threads ; and other 

 filaments, diverging laterally, proceed from each 



439 



440 



ganglion. 



During the progress of growth, the lon- 

 gitudinal cords approach somewhat nearer to each 

 other, but still remain perfectly distinct. 



The next step in the gradation occurs in the 

 P/iyllosoma (Leach), where the ganglia composing 

 each pair in the abdomen and in the head, are 



* These segments are numbered in this and the following figure 

 in their proper order, beginning with that near the head, which is 

 the cephalic ganglion; a is the external antenna; a, the internal 

 antenna ; and e, the eye. 



