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live in holes； for creatures living so cannot possibly be high above
the ground。
But crabs are in nature the oddest of all polypods； they do not
progress forwards except in the sense explained above； they are the
only animals which have more than one pair of leading limbs。 The
explanation of this is the hardness of their limbs； and the fact
that they use them not for swimming but for walking； they always
keep on the ground。 However； the flexion of the limbs of all
polypods is oblique； like that of the quadrupeds which live in
holes…for example lizards and crocodiles and most of the oviparous
quadrupeds。 And the explanation is that some of them in their breeding
periods； and some all their life； live in holes。
17
Now the rest have bandy legs because they are soft…skinned； but
the crayfish is hard…skinned and its limbs are for swimming and not
for walking （and so are not bandy）。 Crabs； too； have their limbs
bent obliquely； but not bandy like oviparous quadrupeds and
non…sanguineous polypods； because their limbs have a hard and
shell…like skin； although they don't swim but live in holes； they live
in fact on the ground。 Moreover； their shape is like a disk； as
compared with the crayfish which is elongated； and they haven't a tail
like the crayfish； a tail is useful to the crayfish for swimming；
but the crab is not a swimming creature。 Further； it alone has its
side equivalent to a hinder part； because it has many leading feet。
The explanation of this is that its flexions are not forward nor its
legs turned in under （bandy）。 We have given above the reason why its
legs are not turned in under； that is the hardness and shell…like
character of its integument。
For these reasons then it must lead off with more than one limb； and
move obliquely； obliquely； because the flexion is oblique； and with
more than one limb； because otherwise the limbs that were still
would have got in the way of those that were moving。
Fishes of the flat kind swim with their heads twisted； as one…eyed
men walk； they have their natural shape distorted。 Web…footed birds
swim with their feet； because they breath the air and have lungs
they are bipeds； but because they have their home in the water they
are webbed； by this arrangement their feet serve them instead of fins。
They have their legs too； not like the rest of birds in the centre
of their body； but rather set back。 Their legs are short； and being
set back are serviceable for swimming。 The reason for their having
short legs is that nature has added to their feet by subtracting
from the length of their limbs； instead of length she gives
stoutness to the legs and breadth to the feet。 Broad feet are more
useful than long for pushing away the water when they are swimming。
18
There is reason； too； for winged creatures having feet； but fish
none。 The former have their home in the dry medium； and cannot
remain always in mid air； they must therefore have feet。 Fish on the
contrary live in the wet medium； and take in water； not air。 Fins
are useful for swimming； but feet not。 And if they had both they would
be non…sanguineous。 There is a broad similarity between birds and
fishes in the organs of locomotion。 Birds have their wings on the
superior part； similarly fish have two pectoral fins； again； birds
have legs on their under parts and near the wings； similarly； most
fish have two fins on the under parts and near the pectorals。 Birds；
too； have a tail and fish a tail…fin。
19
A difficulty may be suggested as to the movements of molluscs；
that is； as to where that movement originates； for they have no
distinction of left and right。 Now observation shows them moving。 We
must； I think； treat all this class as mutilated； and as moving in the
way in which limbed creatures do when one cuts off their legs； or as
analogous with the seal and the bat。 Both the latter are quadrupeds
but misshapen。 Now molluscs do move； but move in a manner contrary
to nature。 They are not moving things； but are moving if as
sedentary creatures they are compared with zoophytes； and sedentary if
classed with progressing animals。
As to right and left； crabs； too； show the distinction poorly； still
they do show it。 You can see it in the claw； the right claw is
larger and stronger； as though the right and left sides were trying to
get distinguished。
The structure of animals； both in their other parts； and
especially in those which concern progression and any movement in
place； is as we have now described。 It remains； after determining
these questions； to investigate the problems of Life and Death。
…THE END…
。