Hydra are solitary animals of the phylum Coelenterata that measure from just a few millimeters in size to more than 3.5 ft (1 m) in length. They are all thin animals that rarely measure more than 0.4 in (1 cm) in diameter. Most are cylindrical in shape, with a broadened basal disk that serves to attach the animal to some firm substrate. Most species are sessile but some can, if conditions require, move over short distances by repeatedly looping the body over onto the substrate. Longer-range movements may be accomplished by releasing their grip on the substrate and rising into the water current.
The main body stalk is a simple, erect tube-like arrangement, at the top of which is the mouth. This is surrounded by a ring of tentacles whose length varies according to the species in question. The body stalk is not encased in a hard protective layer, and the animal is therefore able to flex and bend. The bulk of the body is taken up with the large intestinal cavity.
The tentacles contain a large number of specialized cells called cnidocytes, which contain stinging structures known as nematocysts. The latter vary in shape according to their required purpose. Most have an oval shaped base, attached to a long threadlike structure. When the animal is feeding or is alarmed, the cnidocytes are triggered to release the nematocysts. When the animals is feeding, most of the nematocysts that are released are hollow and elongate, their purpose being to trap and entangle prey. Once this has been completed, the captured prey—often small crustaceans—are grasped by the tentacles and passed down towards the mouth.
In other situations, for example in defense, the nematocysts may be shorter and often bear small spines; some may also contain a toxic substance which is injected into the attacking animal to deter or stun it.
Most hydras reproduce by asexual means through a simple system of "budding-off." In this process, a small extension of the parent animal forms on the body wall. As this grows, a separate mouth and set of tentacles develops until eventually a replicate daughter cell of the parent hydra is produced. When the young animal has fully developed, the two separate and the young hydra drifts off in the current to become established elsewhere. In certain circumstances, particularly where seasonal drought is a regular feature, some species may also practice sexual reproduction which involves the production of a fertilized embryo enclosed in a toughened outer coating. In this state, the young hydra is able to withstand periods of drought, cold, food shortages, or heat. Once conditions normal resume, the outer casing dissolves and the embryo recommences life.