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
amoebæ. 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