Our second ancestral form is also a fresh-water animal, the hydra.  This is a little, vase-shaped animal, which usually lives attached to grass-stems or sticks, but has the power to free itself and hang on the surface of the water or to slowly creep on the bottom.  The mouth is at the top of the vase, and the simple, undivided cavity within the vase is the digestive cavity.  Around the mouth is a ring of from four to ten hollow tentacles, whose cavities communicate freely underneath with the digestive cavity.  Not only is food taken in at the mouth, but indigestible material is thrown out here.  The animal may thus be compared to a nearly cylindrical sack with a circle of tubes attached to it above.  The body consists of two layers of cells, the ectoderm on the outside and the entoderm lining the digestive cavity.  Between these two is a structureless, elastic membrane, which tends to keep the body moderately expanded.

The food is captured by the tentacles; but digestion takes place only partially in the digestive cavity, for each surrounding cell engulfs small particles of food and digests 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.