THE ALIMENTARY CANAL. 
75 
The Protozoa, as the Amoeba and Infusoria, can hardly 
be said to have a digestive canal. The animal is here 
composed of a single cell, in which the food is digest¬ 
ed. The jelly-like Amoeba passes the food through 
the firmer outer layer ( ectosarc ) into the more fluid 
inner part (< endosarc ), where it is digested. The Infu¬ 
soria, 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 excre¬ 
ment 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 (ecto¬ 
derm and endoderm). 
A mouth leads into 
the cavity, and serves 
as well for the out¬ 
let of matter not 
wanted. The endo- 
dermal cells furnish 
, . . , ... Fig. 3S.—Dissected Actinia: a, the thick opaque skin 
the juices by Which consisting of ectoderm, lined with muscular fibres; 
+1.0 fAnrl l'o rliryPQt-pfl c » the tubular tentacles communicating with the 
tile IOOU lb Ul^ebltJU iutergpacegj kj between the membranous vertical 
aild absorb the nil- folds; g, g', orifices in the walls allowing passage 
of respiratory water from one compartment to an- 
tritious portions of other; d, mouth leading to gastric cavity, c. 
