These essential organs must first
develop and the higher must wait. And the inner, digestive, layer of
cells persists in our bodies as the lining of the mid-intestine. We
compared hydra therefore to a little patch of the lining of our
intestine covered with a flake of epidermis; only these layers in
hydra possess powers lost to the corresponding cells of our bodies
in the process of differentiation. Notice, please, that when cell or
organ has once been developed it persists, as a rule, modified, but
not lost. Nature's experiments are not in vain; her progress is very
slow but sure. But hydra has also the promise of better things,
traces of muscular and nervous tissue. There are still no compact
muscles, like our own, much less ganglion or brain or nerve-centre
of individuality. The tissues are diffuse, but they are the
materials out of which the organs of higher animals will
crystallize, so to speak. Notice also that these higher muscles and
nerves are here entirely subservient to, and exist for, digestion
and reproduction.
In the turbellaria the reproductive system has reached a very high
grade of development. It is a complex and beautifully constructed
organ. The digestive system has also vastly improved; it has its own
muscular layers, and often some means of grasping food. But it is
slower in reaching its full development than the reproductive
system.
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