76 
Fie. 37.—Diagrammatic Section of a Sea-urchin 
(Echinus): a, mouth; b, esophagus; ¢, stom- 
ach; d, intestine; , madreporiform tubercle ; 
g, sand-canal; hk, ambulacral ring; *, Polian 
vesicles, which are probably reservoirs of flu- 
id; m, ambulacral tube; 0, anus; p, ambula- 
cra, with their contractile vesicles ; 7, nervous 
ring around the gullet; s, two nervous trunks, 
the right terminating, at anal pole, in a small 
ganglion; t, blood-vascular rings connected by 
v, the contractile heart ; w, two arterial trunks 
radiating from the anal ring; 2, an ovary open- 
ing at the anal pole in a genital plate, y; 2, 
spines, with their tubercles. 
COMPARATIVE ZOOLOGY. 
type. This first rudi- 
ment of an alimentary 
canal begins in a mouth 
well provided with teeth 
and muscles, and ex- 
tends spirally to its 
outlet, which generally 
opens on the upper, or 
opposite,surface. More- 
over, while in many of 
the Worms the canal is 
a simple tube running 
through the axis of the 
cylindrical body from 
oral orifice to anal ap- 
erture, the canal of the 
Sea-urchin shows a dis- 
tinction of parts, fore- 
shadowing the pharynx, gullet, stomach, and intestines of 
‘ Man himself. 
Both mouth and vent have muscles for 
-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- 
lariw), and passed on from one to the other down the side 
of the body, till they are dropped off into the water. 
The next higher modification we find in the Articulate 
subkingdom. 
In the Worms, the digestive tract is either 
a straight, unvarying tube, or divided up into pouches 
(sacculated), as in the Leech, with clusters of little glands, 
called follicles, along the side, which are the rudiments of 
a liver. 
In Myriapods and Larvee, 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.” 
Crustaceans, like the Lobster, 
have a short gullet leading to a large cavity, situated in 
