All the great questions concerning life confront us
here in the lowest term of the animal series, and appear as
insoluble as in the highest.
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.
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