For plants get their food from the
earth by means of their roots; and this food is already elaborated
when taken in, which is the reason why plants produce no excrement,
the earth and its heat serving them in the stead of a stomach. But
animals, with scarcely an exception, and conspicuously all such as are
capable of locomotion, are provided with a stomachal sac, which is
as it were an internal substitute for the earth. They must therefore
have some instrument which shall correspond to the roots of plants,
with which they may absorb their food from this sac, so that the
proper end of the successive stages of concoction may at last be
attained. The mouth then, its duty done, passes over the food to the
stomach, and there must necessarily be something to receive it in turn
from this. This something is furnished by the bloodvessels, which
run throughout the whole extent of the mesentery from its lowest
part right up to the stomach. A description of these will be found
in the treatises on Anatomy and Natural History. Now as there is a
receptacle for the entire matter taken as food, and also a
receptacle for its excremental residue, and again a third
receptacle, namely the vessels, which serve as such for the blood,
it is plain that this blood must be the final nutritive material in
such animals as have it; while in bloodless animals the same is the
case with the fluid which represents the blood.
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