Hydras are fresh-water relatives of sea anemones and jellyfish. Like those other members of the phylum Cnidaria, the hydra's body is flexible. When extended it resembles a cartoonlike tube with a fan or propeller at the top. When contracted in a defensive mode it resembles a bulb with a few hairs on top.
The skin of the hydra's tube body is a clear layer of cells called the epidermis or ectoderm. The inner layer is the endoderm or gastrodermis. As the term "gastro-" implies, the inner layer digests the food. Mesoglea is a loose gel sandwiched between the inner and outer layers. The tentacles are hollow tubes with the same three layers.
No brain as such controls the hydra, but a system of nerves reacts to touch and changes in light and replaces the function of muscles. The fluid tube body extends toward prey and retracts to a round shape when sensing danger. Ordinary motion is accomplished in a somersaulting move called looping. The sticky-foot base, or disk, lets go and reaches beyond the lowered tentacle end of the body and reattaches to the underwater surface it is moving along.
Like other cnidaria, hydras have stinging cells called nematocysts. When ejected from the tentacles, these poisonous cells paralyze passing prey of tiny insects, one-celled animals and other edible organisms, including fish eggs, found in the hydra's temperate and tropical ponds and slow-moving streams.
Undigestible parts of a meal do not get passed along to exit elsewhere from the body. Leftovers are vomited back out the hydra's mouth.
To reproduce, little versions begin to grow from the side of the adult, one or sometimes two at a time. This method is called gemmation or budding. When developed enough the buds separate from the original hydra and go off on their own. Hydras can also reproduce with eggs and sperm, either from individuals with both sexes, called hermaphrodites, or from male and female individuals. Particular times of the year and water temperatures seem to trigger when and how hydras reproduce.
Besides reproduction, hydras have the ability to regrow a lost tentacle and to heal injuries. Regeneration can succeed when only a small part of the original hydra is left to regrow.