44 COMPARATIVE ZOOLOGY. 
tween the periods of eating, and the consequent need of 
a reservoir. For animals, unlike plants, are thrown upon 
their own wits to procure food. . 
The alimentary canal is a continuation of the skin, 
which is reflected inward, as we turn the finger of a 
glove. We find every grade of this reflection, from the 
mere depression in the side of the body of the Ameeba, to 
the sac of the Sea-anemone and the long intestinal tube of 
the Ox. So that food in the stomach is still outside of the 
true body. In fact, there are certain Worms, living inside 
of other animals, which have neither mouth nor stomach, 
but imbibe nourishinent through their skin.” Such a 
‘method of taking food is a link between the plant’s out- 
side mode of nutrition and the internal mode of the ani- 
mal: fundamentally, there is no difference. 
The feeblest sign of a digestive cavity is that extempo- 
rized by the jelly-like Amceba. Wherever a minute sea- 
weed or animalcule happens to come in contact with its 
body, the spot re- 
tracts, forming a de- 
pression, which sinks 
deeper and deeper, 
till the edges meet, 
and the prey is in- 
gulfed. After the 
soluble parts are 
dissolved, the indi- 
gestible residue is 
brought to the sur- 
face by a reverse 
Fra. 36.—Dissected Actinia: a, the thick opaque skin PIOC@SS. 
consisting of ectoderm and: endoderm, lined with C : 
muscular fibres; c, the tubular tentacles communi- A step higher than 
cating with the interspaces, k, between the mem- this is seen in Infu- 
braneous vertical folds, g; g’, orifices in the walls : 
allowing passage of respiratory water from one gsoria and the Hydra, 
compartment to another; d, mouth leading to gas- aa 
tric cavity, e. where a definite oral 
