Hydra are small-bodied freshwater polyps with a tube-like body and a
mouth surrounded by tentacles. The hydra's simple tubular body with its
crown of tentacle has earned it a place in every elementary textbook of
zoology and made it the object of many detailed studies.
The hydra is one of a few freshwater cnadarians. The body of a hydra
is a bag whose wall is made up of two layers of cells separated only by
a very thin layer of non-cellular material. Hydra usually have 5-6
hollow tentacle, but may have up to 12. These tentacles surround the
mouth. The other end of the body is a basal disc which normally anchors
the hydra by a sticky secretion. Both tentacles and body are very
extensible, for the bases of many of the cells are drawn out as muscle
fibers. The nervous system is extremely simple, made up of only a
network of nerve cells. There is no brain of any sorts.
Hydras are primitive marine animals found in ponds.
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