ake 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. But all the muscles are no longer attached to the stomach;
they are beginning to assert their independence, and, in a rude way,
to build a body-wall. But they are in many layers, and run in almost
all directions. Some of these layers will disappear, but the most
important ones, consisting of longitudinal and transverse fibres,
will persist in higher forms. Locomotion by means of these muscles
is slowly coming into prominence. They are no longer merely slaves
of digestion.

But a muscular fibril contracts only under the stimulus of a nervous
impulse. More nerve-cells are necessary to control these more
numerous muscular fibrils. The animal now moves with one end
foremost, and that end first comes in contact with food, hindrances,
or injurious surroundings. Here the sensory cells of feeling and
their nerve fibrils multiply. Remember that these neuro-epithelial
sensory cells are suited to respond not merely to pressure, but to a
variety of the stimuli, chemical, molecular, and of vibration, which
excite our organs of smell, taste, and hearing. Such organs and the
directive eyes appear mainly at this anterior end. But a ganglion
cell sends an impulse to a muscle because it has received one along
a sensory nerve from one or more of these sensory cells. Hence the
ganglion cells will increase in number. The old cobweb-like plexu