THE ALIMENTARY CANAL. 
75 
be said to have a digestive canal. The animal is here 
composed of a single cell, in which the food is digested. 
The jelly-like Amoeba passes the food through the firmer 
outer layer ( ectosarc ) into the more fluid inner part ( endo- 
sarc\ where it is digested. The Infusoria, which have a 
cuticle, and so a more definite form, possess a mouth, or 
opening, into the interior of their cell-body, and at least 
a definite place where the excrement is passed out. But 
we cannot call this cell-cavity a digestive tract. 
In the higher animals, the alimentary canal is a contin¬ 
uation of the skin, which is reflected inward, as we turn 
the finger of a glove. 35 We find every grade of this re¬ 
flection, from the sac of the Hydra to the long intestinal 
tube of the Ox. So that food in the stomach is still out¬ 
side of the true body. 
The simplest form of such a digestive tract is seen in 
the Hydra (Fig. 191). Here the body is a simple bag, 
whose walls are composed of two layers of cells (ectoderm 
and endoderm). A 
mouth leads into the 
cavity, and serves as 
well for the outlet of 
matter not wanted. v 
The endodermal cells 
furnish the juices by 
which the food is di- 
gested and absorb the 
nutritious portions of 
it. There is no rad¬ 
ical difference, how¬ 
ever, between the two 
of respiratory water from one compartment to an- 
Ul J ** ''*** ----I-—- 
the former ectoderm other; d, mouth leading to gastric cavity, e. 
