The hydra has one of the simplest structures of the many-celled animals. Its body is shaped like a thin cylinder about as thick as heavy sewing thread. It is about i- to i-inch long. A hydra may be gray, tan, brown, or bright green. The green hydra gets its color from algae that live in certain cells of its body wall.
One end of a hydra's body attaches to sticks, stones, and water plants. The other end contains the mouth, which is surrounded by five to seven tentacles (tiny arms). The hydra can stretch out or contract its entire body. Sometimes it stretches its tentacles until they look like long, delicate threads. At other times, it pulls in its tentacles. Then its body looks like a tiny egg with a group of knobs on the end. The hydra usually stays attached in one place for some time. But it can move about by slowly somersaulting on the bottom of the pond or lake, or by drifting upside down beneath the water's surface film.
A hydra eats other small water animals that it captures with its tentacles. Each tentacle has tiny cells that contain stinging threads. The hydra drives these threads into its prey, and they give off a poison that paralyzes the victim. Then the tentacles draw the victim into the mouth so the hydra can swallow it. The mouth of the hydra is a small opening leading directly into a large digestive cavity inside the body. The undigested remains pass out through the mouth.
Hydras usually reproduce by budding. Buds are small, knob like growths that appear on the body of the hydra from time to time. As the buds develop, they grow tentacles. When the buds are fully developed, they break off and live as independent hydras. Hydras also may produce eggs and sperm in the body wail. An egg unites with a sperm, grows larger, and eventually separates from the parent and drops into the water. After a while, a small hydra hatches from the egg.
Hydras can regenerate (regrow) parts of their bodies. It is estimated that in a period of several weeks a hydra replaces all the cells in its body. The hydra, therefore, never grows old.
Scientific Classification. Hydras belong to the phylum Coelenterata, and the class Hydroaoa.
