Hydra

Hydra, genus of small, hydrozoan, freshwater animals, 0.25 to 2.5 cm (up to 1 in) long. They are among the simplest in structure of all multicellular animals and are often studied in elementary biology courses. A hydra is a hollow, cylindrical animal, closed at one end, which is known as the foot, and opening at the other into the mouth, which is surrounded by six to ten tentacles. It moves either by gliding on the foot, like a snail, or by slowly somersaulting.

The body of a hydra is made up of two layers of cells: an outside layer, or ectoderm, and an inside layer, or endoderm, separated by a thin layer of secreted jelly (mesoglea). Such an animal is known as a simple polyp. Both layers of the hydra’s cells contain contractile fibres, forerunners of the muscles in higher animals, which relax to allow the animal to expand and are used in locomotion. Interstitial cells, scattered cells found among the ectodermal and endodermal cells, give rise to a network of nerve threads running throughout the entire body and to several testes and ovaries on each animal. Each ovary contains a single egg, and each testis contains several sperm. The eggs are fertilized and develop within the body wall of the parent hydra; the embryos then rupture through the body wall and grow, like buds on a plant, into full-sized adults still attached to the parent. Several such buds may be growing from a hydra at one time; eventually the buds leave the parent and become independent, attaching themselves by their sticky feet to a floating leaf or to a twig.

To capture minute forms of life and to defend itself from larger animals, the hydra is equipped with poison-containing structures in the ectodermal layer. These stinging cells are called cnidocytes (or nematocytes) and the specialized poison capsule and thread they contain is called a nematocyst. Small animals paralysed by the stinging cells are brought into the mouth by the tentacles and then transported to the body cavity; there they are either engulfed by the pseudopodia of amoeba-like cells and digested inside the cells (see Phagocytosis) or decomposed in a fungus-like manner by secretions from all the endodermal cells. Hydras are remarkable for their powers of regeneration. When a hydra is cut into fairly large pieces, each piece develops into a complete individual; small pieces of hydra, when placed in contact with each other, grow together to form a complete individual.

Two species of hydra are common in ponds—a brownish-grey species and a green one. The green colour of the latter is due to an alga that lives in symbiosis within the body cells of the hydra; the alga is passed on to the hydra’s young after entering the sex cells.

Scientific classification: Hydras belong to the phylum Cnidaria. The brownish-grey hydra is classified as Pelmatohydra oligactis and the green hydra as Chlorohydra viridissima.