Moreover, since
this property belongs almost peculiarly to Sanguineous animals, we see
that no Sanguineous animal can progress at more points than four,
and that if it is the nature of anything so to progress at four points
it must of necessity be Sanguineous.
What we observe in the animal world is in agreement with the above
account. For no Sanguineous animal if it be divided into more parts
can live for any appreciable length of time, nor can it enjoy the
power of locomotion which it possessed while it was a continuous and
undivided whole. But some bloodless animals and polypods can live a
long time, if divided, in each of the severed parts, and can move in
the same way as before they were dismembered. Examples are what is
termed the centipede and other insects that are long in shape, for
even the hinder portion of all these goes on progressing in the same
direction as before when they are cut in two.
The explanation of their living when thus divided is that each of
them is constructed like a continuous body of many separate living
beings. It is plain, too, from what was said above why they are like
this. Animals constructed most naturally are made to move at two or
four points, and even limbless Sanguinea are no exception. They too
move by dint of four points, whereby they achieve progression. They go
forward by means of two flexions. For in each of their flexions
there is a right and a left, both before and behind in their flat
surface, in the part towards the head a right and a left front
point, and in the part towards the tail the two hinder points.
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