On The Parts Of Animals   

The anterior legs are in some cases longer than the others, that they
may serve to wipe away any foreign matter that may lodge on the
insect's eyes and obstruct its sight, which already is not very
distinct owing to the eyes being made of a hard substance. Flies and
bees and the like may be constantly seen thus dressing themselves with
crossed forelegs. Of the other legs, the hinder are bigger than the
middle pair, both to aid in running and also that the insect, when it
takes flight, may spring more easily from the ground. This difference
is still more marked in such insects as leap, in locusts for instance,
and in the various kinds of fleas. For these first bend and then
extend the legs, and, by doing so, are necessarily shot up from the
ground. It is only the. hind legs of locusts, and not the front ones,
that resemble the steering oars of a ship. For this requires that the
joint shall be deflected inwards, and such is never the case with the
anterior limbs. The whole number of legs, including those used in
leaping, is six in all these insects.
Part 7
In the Testacea the body consists of but few parts, the reason being
that these animals live a stationary life. For such animals as move
much about must of necessity have more numerous parts than such as
remain quiet; for their activities are many, and the more diversified
the movements the greater the number of organs required to effect
them. Some species of Testacea are absolutely motionless, and others
not quite but nearly so. Nature, however, has provided them with a
protection in the hardness of the shell with which she has invested
their body. This shell, as already has been said, may have one valve,
or two valves, or be turbinate. In the latter case it may be either
spiral, as in whelks, or merely globular, as in sea-urchins. When it
has two valves, these may be gaping, as in scallops and mussels, where
the valves are united together on one side only, so as to open and
shut on the other; or they may be united together on both sides, as in
the Solens (razor-fishes). In all cases alike the Testacea have, like
plants, the head downwards. The reason for this is, that they take in
their nourishment from below, just as do plants with their roots. Thus
the under parts come in them to be above, and the upper parts to be
below. The body is enclosed in a membrane, and through this the animal
filters fluid free from salt and absorbs its nutriment. In all there
is a head; but none of the parts, excepting this recipient of food,
has any distinctive name.
Part 8
All the Crustacea can crawl as well as swim, and accordingly they are
provided with numerous feet. There are four main genera, viz. the
Carabi, as they are called, the Astaci, the Carides, and the Carcini.
In each of these genera, again, there are numerous species, which
differ from each other not only as regards shape, but also very
considerably as regards size. For, while in some species the
individuals are large, in others they are excessively minute. The
Carcinoid and Caraboid Crustacea resemble each other in possessing
claws. These claws are not for locomotion, but to serve in place of
hands for seizing and holding objects; and they are therefore bent in
the opposite direction to the feet, being so twisted as to turn their
convexity towards the body, while their feet turn towards it their
concavity. For in this position the claws are best suited for laying
hold of the food and carrying it to the mouth. The distinction between
the Carabi and the Carcini (Crabs) consists in the former having a
tail while the latter have none. For the Carabi swim about and a tail
is therefore of use to them, serving for their propulsion like the
blade of an oar. But it would be of no use to the Crabs; for these
animals live habitually close to the shore, and creep into holes and
corners. In such of them as live out at sea, the feet are much less
adapted for locomotion than in the rest, because they are little given
to moving about but depend for protection on their shell-like
covering. The Maiae and the crabs known as Heracleotic are examples of
this; the legs in the former being very thin, in the latter very