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History of Animals   

of feeding; for as they get their food in the water they cannot but
take in water along with their food, and if they take in water they
must be provided with some organ for discharging it. Those blooded
animals, then, that use water for a purpose analogous to respiration
are provided with gills; and such as take in water when catching their
prey, with the blow-hole. Similar remarks are applicable to molluscs
and crustaceans; for again it is by way of procuring food that these
creatures take in water.
Aquatic in different ways, the differences depending on bodily
relation to external temperature and on habit of life, are such
animals on the one hand as take in air but live in water, and such
on the other hand as take in water and are furnished with gills but go
upon dry land and get their living there. At present only one animal
of the latter kind is known, the so-called cordylus or water-newt;
this creature is furnished not with lungs but with gills, but for
all that it is a quadruped and fitted for walking on dry land.
In the case of all these animals their nature appears in some
kind of a way to have got warped, just as some male animals get to
resemble the female, and some female animals the male. The fact is
that animals, if they be subjected to a modification in minute organs,
are liable to immense modifications in their general configuration.
This phenomenon may be observed in the case of gelded animals: only
a minute organ of the animal is mutilated, and the creature passes
from the male to the female form. We may infer, then, that if in the
primary conformation of the embryo an infinitesimally minute but
absolutely essential organ sustain a change of magnitude one way or
the other, the animal will in one case turn to male and in the other
to female; and also that, if the said organ be obliterated altogether,
the animal will be of neither one sex nor the other. And so by the
occurrence of modification in minute organs it comes to pass that
one animal is terrestrial and another aquatic, in both senses of these
terms. And, again, some animals are amphibious whilst other animals
are not amphibious, owing to the circumstance that in their
conformation while in the embryonic condition there got intermixed
into them some portion of the matter of which their subsequent food is
constituted; for, as was said above, what is in conformity with nature
is to every single animal pleasant and agreeable.
Animals then have been categorized into terrestrial and
aquatic in three ways, according to their assumption of air or of
water, the temperament of their bodies, or the character of their
food; and the mode of life of an animal corresponds to the category in
which it is found. That is to say, in some cases the animal depends
for its terrestrial or aquatic nature on temperament and diet
combined, as well as upon its method of respiration; and sometimes
on temperament and habits alone.
Of testaceans, some, that are incapable of motion, subsist on
fresh water, for, as the sea water dissolves into its constituents,
the fresh water from its greater thinness percolates through the
grosser parts; in fact, they live on fresh water just as they were
originally engendered from the same. Now that fresh water is contained
in the sea and can be strained off from it can be proved in a
thoroughly practical way. Take a thin vessel of moulded wax, attach
a cord to it, and let it down quite empty into the sea: in twenty-four
hours it will be found to contain a quantity of water, and the water
will be fresh and drinkable.
Sea-anemones feed on such small fishes as come in their way. The
mouth of this creature is in the middle of its body; and this fact may
be clearly observed in the case of the larger varieties. Like the
oyster it has a duct for the outlet of the residuum; and this duct
is at the top of the animal. In other words, the sea-anemone
corresponds to the inner fleshy part of the oyster, and the stone to
which the one creature clings corresponds to the shell which encases
the other.
The limpet detaches itself from the rock and goes about in quest

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