ests them within itself. The
entodermal cells behave in this respect much like a colony of
amoebae. 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.

The reproductive organs for the production of eggs and spermatozoa
form little protuberances on the outside of the body below the
tentacles. But hydra reproduces mostly by budding; new individuals
growing out of the side of the old one, like branches from the trunk
of a tree, but afterward breaking free and leading an independent
life. There are special forms of cells besides those described;
nettle cells for capturing food, interstitial cells, etc., but these
do not concern us.

The distance from the single-celled amoeba to hydra is vast,
probably really greater than that between any other successive terms
of our series. It may therefore be useful to consider one or two
intermediate forms and the parallel embryonic stages of higher
animals, and to see how the higher many-celled animal originates
from the unicellular stage.

The amoeba is an illustration of a great kingdom of similar,
practically unicellular forms, which have played no unimportant part
in the geol