Hydra is a genus of small multicellular animals that live in freshwater ponds throughout the world. It's tiny, just barely visible with the unaided eye. It's also sessile, which means it spends most of its life stuck to some piece of dead vegetation or other detritus that floats around in the pond or rests on its bottom.
The hydra owes its name to the shape of its body, which resembles, by a reasonably poetic stretch of the imagination, the Hydra of classical Greek mythology. Most of its body is a thin, stalk-like tube, which can contract to a thick stub or extend out to a long and thin stem. The base of the tube has an organ for adhering to something for stability. At the other end of the tube is a head thing that features six tentacles that branch out from the top of the body tube in radial symmetry. The hydra's mouth, its only opening, is a simple hole located at the center of the part where the tentacles attach to the body.
The tentacles are handy for grabbing food and urging it into the hydra's mouth opening. The grabbing is aided by the tentacles having very many tiny nematocysts, which are spring-loaded poison barbs that shoot out and harpoon prey. If you've ever felt the sting of a jellyfish, then you know what nematocysts are about.
It's got the nerve
The hydra is extremely simple as animals go. The most important way in which it differs from the even simpler and much less interesting sponges is that the hydra has a true nervous system; it is the first organism on the ladder of animal complexity to have one. This evolutionary advance gifts the hydra with the ability to move in complex purposeful ways. It can sense when food gets stuck on its tentacles and the tentacles then move in cooperation to get the food into the mouth hole.
It gets around
Another impressive trick enabled by the nervous system is mobility. While the hydra seldom roams, it can cut loose from its mooring and move to another location when conditions require. It accomplishes that migration by a somersaulting action that is quite like a gymnast doing handsprings, albeit much more slowly and much less gracefully.
There are male hydras and female hydras, but their sexual activities are sure to disappoint you sweaty-palmed, microscope-peering voyeurs. To begin with, hydra reproduce by budding most of the time, which involves no sex at all. Budding happens when some cells on the stalk just start growing into a little baby hydra that eventually pops off on its own. But the actual sex, when it happens, is even less interesting than that. Totally SFW. Males develop spermaries, which generate sperm and spew them out into the water. Females develop ovaries that produce eggs and spew them out into the water. When sperm and egg defy the odds to meet and hit it off, a zygote results and that develops into a new hydra. This life cycle is unique among the hydrazoa in that there is no intermediate medusa (jellyfish) stage.
An interesting ability of the hydra body is regeneration. Strangely like its mythical namesake, if any bit of a hydra is cut off, it grows back. That bit can be a tentacle, the whole head, or even most of the stalk. Not only that, the hydra stem cells renew themselves, forever. Thus, these creatures do not age like other animals do, so they never die, at least not until they get eaten by a flatworm, crustacean, or aquatic insect of one kind or another.
Phylum: Cnidaria (Coelenterata)
Hydra stem cells (scientific paper)
Hydra motion with evident budding (video)