Hydra (invertebrate)

    The hydra is a minute fresh-water polyp typical of one of the lower forms of animals famous for being able to reproduce any part of its body. If cut into pieces each piece will become a perfect polyp. This is due to lack of differentiation of tissues, there being no distinct nervous or circulatory system. The hydra is visible to the naked eye—it is of the size of sewing cotton—and lives on the under side of submerged stems and leaves. Its tube-shaped body is very retractile; one end of it holds onto the stem or leaf, while the other swings to a new position. In this way it accomplishes a slow movement which enables it to forage for its food of tiny crustaceans, which paralyzed when in contact with the five or eight long tentacles, armed with nettling cells buried in the skin, with which the mouth at the upper end of the body is surrounded.