THE ALIMENTARY CANAL 



2/9 



constriction and expansion ; and, as the vent is on the 



summit of the shell, and the latter is covered with 



spines, the ejected 



particles are seized by 



delicate forks (pedicel- 



larice\ and passed on 



from one to the other 



down the side of the 



body, till they are 



dropped off into the 



water. 91 



The worms present 

 us with a great range 



Of Structure in the 



digestive tract. It is 

 sometimes almost as 



Simple aS that Of the 



a a mere SaC. 



f^rt-h wnrm hv<z x 



n nas a 



tube running straight 

 through the body, di- 

 vided into pharynx, esophagus, crop, gizzard, and saccu- 

 lated intestine (Fig. 52). The leech has large sacs on 

 each side of the intestine. The sea worms, like Nereis, 

 have the pharynx armed with teeth, and some have 

 glandular coeca attached to the intestine. The plan is 

 that of a straight tube extending from mouth to anus. 

 In myriapods and larvae of insects, the same general 

 plan is continued, the canal passing in a straight line 

 from one extremity to the other, but showing a division 

 into gullet, stomach, and intestine. 92 Crustacea, like 

 the lobster, have a short gullet leading to a large 

 cavity, situated in the front of the animal, which is a 

 gizzard, rather than stomach, as it has thick muscular 

 walls armed with teeth. A well-marked constriction 



FIG. 237. Diagrammatic Section of a Sea Urchin 

 (Echinus): a, mouth; b, esophagus; c, stom- 

 ach; d, intestine; f t madreporiform tubercle; 

 g, stone canal; h, ambulacral ring; k, Polian 

 vesicles, which are probably reservoirs of fluid; 

 m, ambulacral tube; o, anus; p, ambulacra, with 

 their contractile vesicles; r, nervous ring around 

 the gullet; s, two nervous trunks, the right termi- 

 nating, at anal pole, in an eye; t, blood-vascular 

 rings connected by v, the intestinal blood vessel; 

 iv, two arterial trunks radiating from the anal 

 ring; x, an ovary opening at the anal pole in a 

 genital plate, y; z, spines, with their tubercles. 



