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 



tho vpnt k on thp mini- Fie. 39._Diagrammatic Section of a Sea-nrchin 



(Echinus): a, month ; b, oesophagus; c, stom- 



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 larvse 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/ 8 Crustacea, like 



ach ; (f, intestine: /, madreporitbrm tubercle; 

 (j, stone-canal; h, ambnlacral ring; fc, Polian 

 vesicles, which are probably reservoirs of fluid; 

 m, ambnlacral tube; o, anus; jo, ambulacra, 

 with their contractile vesicles; r, nervous ring 

 around the gullet; s, t\vo 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- 

 diatir.g from the anal ring; x, an ovary open- 

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

 spines, with their tubercles. 



