THE ALIMENTARY CANAL. 



77 



surface. Moreover, while in some of the Worms the canal 

 is a simple tube running through the axis of the cylindri- 

 cal body from oral ori- 

 fice to anal aperture, the 

 canal of the Sea-urchin 

 shows a distinction of 

 parts, foreshadowing the 

 pharynx, gullet, stom- 

 ach, and intestines. Both 

 mouth and vent have 

 muscles for constriction 

 and expansion; and, as 

 the vent is on the sum- 

 mit of the shell, and the 

 latter is covered with 

 spines, the ejected par- 

 ticles are seized by del- 

 icate forks {pedicella- 

 rice), and passed on from 

 one to the other down 

 the side of the body, till they are dropped off into the 

 water. 37 



The Worms present us with a great range of structure 

 in the digestive tract. It is sometimes almost as simple 

 as that of the Hydra — a mere sac. The Earth-worm has 

 a tube running straight through the body, divided into 

 pharynx, oesophagus, crop, gizzard, and sacculated intes- 

 tine. The Leech has large sacs on each side of the intes- 

 tine. The Sea-worms have the pharynx armed with teeth, 

 and some have glandular cceca 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 divis- 

 ion into gullet, stomach, and intestine. 38 Crustacea, like 



Fie 39. — Diagrammatic Section of a Sea-urchin 

 (Echinus) : a, mouth ; 5, oesophagus ; c, stom- 

 ach ; <?, intestine ; /, 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 terminating, at anal pole, in a small gan- 

 glion ; t, blood-vascular rings connected by v, 

 the contractile heart ; w, two arterial trunks ra- 

 diating from the anal ring ; x, an ovary open- 

 ing at the anal pole in a genital plate, y; z, 

 spines, with their tubercles. 



