Hydras

Hydra is a genus of small freshwater cnidarians in the phylum Cnidaria, class Hydrozoa. Hydra species are sessile polyp forms, typically a tubular body with a mouth at the top surrounded by a circle of tentacles. They lack a free-swimming medusa stage. The body wall consists of two cell layers—the epidermis and the gastrodermis—separated by a gelatinous mesoglea. Hydra feed by capturing prey with their tentacles and delivering nematocysts, then ingesting it into a central gastrovascular cavity that serves both digestion and distribution of nutrients.

Hydra reproduce primarily by asexual budding, in which new individuals form as outgrowths from the parent. Sexual reproduction also occurs in some species, usually under adverse conditions. The nervous system is a diffuse nerve net; contraction and movement are coordinated by simple neural circuits and epitheliomuscular cells. Many species can regenerate completely from small fragments, and some have cells capable of changing their fate, a property that has made Hydra a long-standing model in developmental biology. Some species, such as Hydra viridissima, harbor endosymbiotic algae and benefit from photosynthetic products.

Hydra also refers to a mythological creature from Greek folklore: the Lernaean Hydra, a multi-headed serpent slain by Heracles. In myth, cutting off a head often produced two more, a problem overcome by cauterizing the necks as they were removed. The term Hydra is also used metaphorically to denote a problem that becomes more complex when it is attacked.