THE ANNELIDES OR ENTOMOZOARIA. 



403 



509. The tendency to repetition, 

 so remarkable in the tegumentary 

 skeleton of the annelides, extends to 

 the muscles, nervous system, and 

 others. In general each segment has 

 a pair of nervous ganglions ; all these 

 are united to each other by a double 

 chain of communicating filaments 

 occupying the median line of the body 

 near its ventral aspect (Fig. 162). 

 In most of the inferior articulated 

 animals, and in the more elevated, 

 but whose development is not finished, 

 these ganglions are almost all equal, 

 and form, thus connected together, 

 two chains resembling knotted cords 

 from one end of the body to the other 

 (Fig. 414) ; but, as we ascend in the 

 scale, we find a disposition in these 

 ganglions to a fusion, in the lateral 

 and longitudinal sense (Fig. 415), so 

 as to form a mass. In crabs, this 

 centralization is carried so far as to 

 form but two nervous masses for all 

 the rings or segments of the body, 

 one situated in the head, the other 

 in the thorax ; but even in this case 

 the nervous collar encircling the 

 gullet is always found, so that two 

 such masses seem the maximum of 

 nervous fusion. In the mollusca we 

 also find the nervous collar; but 

 the ventral or post-oasophagal gan- 





Fig. 414. Anatomy of 

 the Sphinx.* 



their course encircling the gullet; c, first pair of post-cesophagal ganglions 



mity of the abdomen in the larva. 



D D 2 



