Of animals otherwise, a great many have, besides the organs
above-mentioned, an organ for excretion of the sperm: and of animals
capable of generation one secretes into another, and the other into
itself. The latter is termed 'female', and the former 'male'; but some
animals have neither male nor female. Consequently, the organs
connected with this function differ in form, for some animals have a
womb and others an organ analogous thereto.

   The above-mentioned organs, then, are the most indispensable parts
of animals; and with some of them all animals without exception, and
with others animals for the most part, must needs be provided.

    One sense, and one alone, is common to all animals-the sense of
touch. Consequently, there is no special name for the organ in which
it has its seat; for in some groups of animals the organ is identical,
in others it is only analogous.

                                   4

    Every animal is supplied with moisture, and, if the animal be
deprived of the same by natural causes or artificial means, death
ensues: further, every animal has another part in which the moisture
is contained. These parts are blood and vein, and in other animals
there is something to correspond; but in these latter the parts are
imperfect, being merely fibre and serum or lymph.

    Touch has its seat in a part uniform and homogeneous, as in the
flesh or something of the kind, and generally, with animals supplied
with blood, in the parts charged with blood. In other animals it has
its seat in parts analogous to the parts charged with blood; but in
all cases it is seated in parts that in their texture are homogeneous.

    The active faculties, on the contrary, are seated in the parts
that are heterogeneous: as, for instance, the business of preparing
the food is seated in the mouth, and the office of locomotion in the
feet, the wings, or in organs to correspond.

    Again, some animals are supplied with blood, as man, the horse,
and all such animals as are, when full-grown, either destitute of
feet, or two-footed, or four-footed; other animals are bloodless, such
as the bee and the wasp, and, of marine animals, the cuttle-fish,
the crawfish, and all such animals as have more than four feet.

                                   5

    Again, some animals are viviparous, others oviparous, others
vermiparous or 'grub-bearing'. Some are viviparous, such as man, the
horse, the seal, and all other animals that are hair-coated, and, of
marine animals, the cetaceans, as the dolphin, and the so-called
Selachia. (Of these latter animals, some have a tubular air-passage
and no gills, as the dolphin and the whale: the dolphin with the
air-passage going through its back, the whale with the air-passage
in its forehead; others have uncovered gills, as the Selachia, the
sharks and rays.)

    What we term an egg is a certain completed result of conception
out of which the animal that is to be develops, and in such a way that
in respect to its primitive germ it comes from part only of the egg,
while the rest serves for food as the germ develops. A 'grub' on the
other hand is a thing out of which in its entirety the animal in its
entirety develops, by differentiation and growth of the embryo.

    Of viviparous animals, some hatch eggs in their own interior,
as creatures of the shark kind; others engender in their interior a
live foetus, as man and the horse. When the result of conception is
perfected, with some animals a living creature is brought forth,
with others an egg is brought to light, with others a grub. Of the
eggs, some have egg-shells and are of two different colours within,
such as birds' eggs; others are soft-skinned and of uniform colour, as
the eggs of animals of the shark kind. Of the grubs, some are from the
first capable of movement, others are motionless. However, with regard
to these phenomena we shall speak precisely hereafter when we come
to treat of Generation.

    Furthermore, some animals have feet and some are destitute
thereof. Of such as have feet some animals have two, as is the case
with men and birds, and with men and birds only; some have four, as
the lizard and the dog; some have more, as the centipede and the
bee; but allsoever that have feet have an even number of them.

    Of swimming creatures that are destitute of feet, some have
winglets or fins, as fishes: and of these some have four fins, two
above on the back, two below on the belly, as the gilthead and the
basse; some have two only,-to wit, such as are exceedingly long and
smooth, as the eel and the conger; some have none at all, as the
muraena, but use the sea just as snakes use dry ground-and by the way,
snakes swim in water in just the same way. Of the shark-kind some have
no fins, such as those that are flat and long-tailed, as the ray and
the sting-ray, but these fishes swim actually by the undulatory motion
of their flat bodies; the fishing frog, however, has fins, and so
likewise have all such fishes as have not their flat surfaces
thinned off to a sharp edge.

    Of those swimming creatures that appear to have feet, as is the
case with the molluscs, these creatures swim by the aid of their
feet and their fins as well, and they swim most rapidly backwards in
the direction of the trunk, as is the case with the cuttle-fish or
sepia and the calamary; and, by the way, neither of these latter can
walk as the poulpe or octopus can.