NERVOUS SYSTEM. 201 



Many of the lower amphipoda and isopoda have the segments 

 nearly equally developed from the anterior to the posterior 

 extremity of the trunk, and this equal development is seen 

 also in the nervous columns and ganglia of these segments, 

 as shewn in the annexed figure of those of the talitrus locust a, 

 or common sand-hopper (Fig. 86. A.) The slender longitudi- 

 nal columns and the minute ganglia along their course here 

 remain distinctly separated from each other by a small space 

 on the median plain ; the ganglia are nearly of the same 

 size from the first pair (1,) above the oesophagus (a,) to the 

 caudal pair (11,) and the pairs are almost equidistant along 

 the whole trunk, in a longitudinal direction. This simplest 

 adult form of the nervous system shown by Audouin and 

 Edwards in the talitrus, has been pointed out likewise by 

 Rathke in the idotea, and the same is seen through the trans- 

 parent bodies of many other minute isopods. The same 

 form of the nervous columns is seen in the highest Crustacea 

 while yet in their embryo condition in the ovum. In the 

 short and broad trunk of the cymothoa where the legs are still 

 equally developed along the whole sides of the body, the 

 nervous columns (Fig. 86. B,) have already approximated to 



FIG. 86. 



