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 orb 
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 
f-Lp vpnt is nn Hip snm Fie. 39.— Diagrammatic Section of a Sea-urcliin 
me vent IS on me sum {Echinm): a >mou th; b, oesophagus; c, stom¬ 
ach ; cl, intestine; /, madreporiform tubercle ; 
< 7 , 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. 
mit of the shell, and the 
latter is covered with 
spines, the ejected par¬ 
ticles are seized by del¬ 
icate forks {jpedicella- 
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 coeca 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 . 38 Crustacea, like 
