76 
COMPARATIVE ZOOLOGY. 
lias digested the food and the former endoderm has taken 
on the functions of the outer layer. The Polyps have 
also but one external opening; but from this hangs down 
a short tube, open at both ends, reaching about half-way 
to the bottom of the body-cavity. Such an arrangement 
would be represented by a bottle with its neck turned 
inward. In this suspended sac, which is somewhat con¬ 
stricted at the extremities, digestion takes place; but the 
product passes freely into all the surrounding chambers, 
along with the water for respiration. The Medusae, or 
Jelly-fishes, preserve the same type of a digestive appara¬ 
tus ; but the sac is cut off from the general cavity, and 
numerous canals radiate from it to a circular canal near 
the margin of the disk. In the Star-fishes (Fig. 126), we 
find a great advance. The sac-like stomach sends off two 
glandular branches to each arm, which doubtless furnish 
a fluid to aid in digestion (so-called hepatic coeca). There 
is also an anus present in some forms, but it hardly serves 
to pass off the waste matter. 
Thus far we have seen but one opening to the digestive 
cavity, rejected portions returning by the same road by 
which they enter. But a true alimentary canal should 
have an anal aperture distinct from the oral. The sim¬ 
plest form of such a canal is exhibited by the Sponge, in 
its system of absorbent pores for the entrance of liquid, 
and of several main channels for its discharge. The 
apparatus, however, is not marked off from the general 
cavity of the body, and digestion is not distinct from cir¬ 
culation. 36 
The Sea-urchin presents us with an important advance 
—one cavity with two orifices; and the complicated ap¬ 
paratus of higher animals is but the development of this 
type. This alimentary canal begins in a mouth well pro¬ 
vided with teeth and muscles, and extends spirally to its 
outlet, which generally opens on the upper, or opposite, 
