76 
COMPARATIVE ZOOLOGY. 
it. The Polyps have also but one external opening; but 
from this hangs down a short tube, open at both ends, 
raaching 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 suspend¬ 
ed sac, which is somewhat constricted at the extrem¬ 
ities, digestion takes place; but the product passes freely 
into all the surrounding chambers, along with the water 
for respiration (Fig. 38). The Medusae, or Jelly-fishes, 
preserve the same type of a digestive apparatus; but 
the sac is cut off from the general cavity, and numer¬ 
ous canals radiate from it to a circular canal near the 
margin of the disk (Fig. 196). 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 cceca). There is also an anus present in 
some forms, but it hardly serves to pass off the waste 
matter. 
Thus far w r e 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, 
