THE PHYLUM ANNELIDA 



dorsal being the larger. Toward the anterior end of the body the parapodia 

 are reduced in size; and toward the posterior end, where new somites are 

 added as the worm grows, they are not fully developed. Parapodia are found 

 on all segments except those surrounding the mouth and the anus. Nereis is 

 thus composed of a series of similar structural units repeated throughout the 

 body, but with certain specializations at each end. 



Internally, the digestive tract is a straight tube extending from mouth to 

 anus. The buccal region and pharynx are eversible, and the pharynx, bearing 

 two jaws and numerous small denticles, constitutes a proboscis used in seizing 

 prey. A narrow esophagus, receiving ducts from a pair of glands, follows the 

 pharynx; from about the twelfth somite posteriorly, the tract consists of a 

 simple tube, the stomach-intestine. Surrounding the digestive tract is a well- 

 defined coelomic body cavity, divided by septa extending transversely from 

 body wall to gut. The compartments thus formed constitute the coelom; they 

 are further divided into lateral halves by mesenteries which extend in the 

 median plane dorsal and ventral to the gut. The coelomic cavities contain a 

 fluid with amoeboid cells, comparable with the lymph of a vertebrate. 



The circulatory system is of the "closed" type, consisting of a dorsal and a 

 ventral longitudinal blood vessel, with lateral branches to capillary beds in the 

 bodv wall and the wall of the intestine. Capillaries are particularly numerous 



Fig. 14.2. Specializations of the anterior end of \ereu inrens. A. dorsal view; note the 

 two prostomial and eight peristomial tentacles, the two blunt palps, and the eves. B, ventral 

 view of a specimen with pharynx and jaws everted, as in seizins; prey. (Photographs bv Bassett 

 Maguire, Jr.) 



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