The cells of both layers have at their bases long muscular
fibrils, those of the ectodermal cells running longitudinally, those
of the entoderm transversely. The animal can thus contract its body
in both directions, or, if the body contain water and the transverse
muscles are contracted, the pressure of the water lengthens the body
and tends to extend the tentacles.
On the outside of the elastic membrane, just beneath the ectoderm,
is a plexus or cobweb of nervous cells and fibrils. As in every
nervous system, three elements are here to be found. 1. An afferent
or sensory nerve-fibril, which under adequate stimulus is set in
vibration by some cell of the epidermis or ectoderm, which is
therefore called a sensory cell. 2. A central or ganglion
cell, which receives the sensory impulse, translates it into
consciousness, and is the seat of whatever powers of perception,
thought, or will the animal possesses. This also gives rise to the
efferent or motor impulses, which are conveyed by (3) a motor fibril
to the corresponding muscle, exciting its contraction. But there are
also nerve-fibrils connecting the different ganglion cells, so that
they may act in unison. In the higher animals we shall find these
central or ganglion cells condensed in one or a few masses or
ganglia. But here they are scattered over the whole surface of the
elastic supporting membrane.
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