Hydra

Hydras are amazing little predatory fresh-water creatures. Although to the naked eye they seem like they don't do much more than just sit there or just float around, they are actually quite active in catching prey with their lengthy tentacles. They are a part of the phylum Cnidaria and the class Hydrozoa. They are found in non-polluted ponds, lakes, and streams, and can be caught by sweeping a net through weedy areas of a pond, lake, or stream. They're only a few millimeters long, and thus are best viewed through a microscope. They have very fast regenerative capabilities, which means that if something happens to a part of their body, they can grow it back faster than most animals.

Anatomy
There are about 3,500 different known species of the hydrozoa. Most hydras stay in the polyp stage, but the free floating medusa stage is found in some cases. Hydras have different shapes, sizes, and looks. They can either be a skinny floating stick with tentacles coming out of the top of the hydra, or they can have the medusa stage, with the floating head, body, and tentacles. Hydras' tentacles are used for protection, movement, and eating. They have stinging cells called nematocysts, mainly to kill organisms for food. Hydras can come in different colors, depending on the species of hydrozoa. Hydras have radial symmetry which means it can be divided into two identical halves by any cut that passes through the longitudinal axis.

Reproduction
All Hydras are considered hermaphrodites. Most Hydras reproduce asexually when food is plentiful. They reproduce by producing buds in their body walls that grow to miniature adults and then just break off when they are mature. Only when weather conditions are harsh, like before a cold winter, do some hydras reproduce sexually. In this case, they produce unfertilized eggs that are then fertilized by sperm from the testes that form on the outer surface of the hydra. The fertilized eggs are secreted by a thick outer coating, then the adult hydra dies. The eggs then fall/float to the bottom of the lake or pond to wait for better, less harsh weather where it will then hatch and grow into a mature adult hydra.

Ecology
Hydras are known to live in ponds or freshwater areas, mostly in the swampy parts. Some hydras are sessile and attach to twigs, stones, debris, or vegetation. On occasion hydras will move spontaneously. Some hydras mostly move by looping and somersaulting themselves by attaching the tentacle ends and moving themselves along. Their behavior is characterized by their mechanical nature, great independence of parts, lack of integration, and lack of exact responses. Hydras are all carnivorous and eat lots of different kinds of small metazoans, including annelids, copepods, cladocerans, and insects. Hydras capture their food by paralyzing and killing their food by nematocysts. Then they use their tentacles to bring the dead organism to the mouth. Their mouths are star shaped or circular.

Related References

 * Hydra (genus)
 * Introduction To The Hydrozoa
 * The Hydrazoa Directory
 * Hydra Oligactis