THE ALIMENTARY CANAL. 75 
orifice, or mouth, leads to a permanent body-cavity, and 
serves both for the inlet of food and the outlet of mat- 
ters not wanted. These animals may be likened to a tube 
or bag with one opening. There is no great difference be- 
tween the membrane which lines this “stomach” and that 
which clothes the body; for the Hydra has been turned 
inside out, and digested as well. 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 Medusz, or 
Jelly-fishes, preserve the same type of a digestive appa- 
ratus; 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, the sac sends 
off two branches, or canals, to each ray. But these radi- 
ating canals serve a double purpose, for they not only carry 
nutritive matter, but bring back the excretions. 
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 ap- 
paratus, however, is not marked off from the general cay- 
ity of the body, and, as in the preceding cases, digestion is 
not distinct from circulation.” 
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 
