The gastroderm is rich of glandular cells producing digestive enzymes which go into the gastric cavity. The body as well as the tentacles is formed by only two layers of cells (it is a diploblastic organism), the ectoderm, external, and the gastroderm, inner, separated by a non cellular gelatinous layer called mesoglea. The lower extremity of the body, the “foot”, produces an adhesive secretion allowing the anchorage to the substratum. Through the hypostome, which is the only opening, are ingested the preys and are egested the digestion residuals.
At the centre of the hypostome opens the mouth, strongly extensible, which leads to the coelenterons, the cavity occupying the whole length of the body, with digestive function. The hydra is a small fresh-water polyp with an about 10 mm long tubular body when extended and a “head”, called hypostome, equipped with a variable number of tentacles (up to 12, more frequently 5 or 7). The predators of the hydras include plathelminths, crustaceans and aquatic insects. The nourishing substances the hydra gets from the symbiont help remarkably its metabolism and this species is a less greedy predator, and its nematocysts are smaller than those non symbiont. The algae are hosted inside the cells of the gastroderm. This species is very interesting as it presents an obliged symbiosis with a unicellular green alga, of the genus Chlorella. In Italy, besides Hydra vulgaris is present Hydra oligactis (Pallas 1766) and the Green hydra ( Hydra viridissima, Pallas, 1766, sinonimi Chlorohydra vividissima Pallas, 1766, Hydra viridisLinnaeus, 1767). Its small size and the fact that, if upset, it contracts further reducing the size render very difficult its finding.
Despite its diffusion, the hydra is very difficult to observe in nature, since it lives anchored usually on the aquatic vegetation. In Italy, the species is relatively common, even if the pollution has strongly reduced its presence. The hydra lives in clean and well oxygenated waters of ponds, small lakes and also resurgences. To the group Hydra vulgaris belong numerous species and subspecies of uncertain distinction present in all the continents. The genus Hydra is diffused in the fresh waters of the temperate and tropical zones of all the continents. Conversely, the term cnidarians and related cnidocytes and cnidocysts come from cnida “κνιδη”, meaning nettle. “Hydra”, in Greek “ὕδρα” means water snake, from “ὕδωρ”, water.
On the basis of Trembley’s observations and the name “hydra” he attributed to its seven-headed specimen, Linnaeus, in 1758, assigns the name of Hydra to the genus. Equally poisonous is the bite of the stinging cells of the hydra, capable to paralyze in a few instants preys much greater than the hydra itself. The Hydra of Lerna was very poisonous: its breath was deadly and the blood was used by Heracles for poisoning his arrows (and by Deïanira for poisoning the famous shirt (the shirt of Nessus) which eventually led Heracles to his death). The analogy between the mythological monster and our Hydra goes further. During his experiments on the regeneration, Trembley gets a specimen with seven heads which it names “Hydra” in memory of the mythological Lernaean Hydra, the monster with many heads (nine after most of versions), capable to regenerate the cut heads, that Hercules faced and killed during the second of his famous “labours” (in this case, played unfairly as he was helped by his nephew Iolaus: Hercules did cut the heads and Iolaous cauterized the wound with a torch). Trembley carried out most of his observations on the green hydra ( Hydra viridissima) and recognized its belonging to the animal kingdom. The first report about the Hydra ( Hydra vulgarisPallas 1766), phylum Cnidaria, class Hydrozoa, family Hydridae, is probably to be ascribed to the Dutch microscopist Anton van Leeuwenhoek (1702-1703) but by sure the most accurate and complete study is that done by the Swiss Abraham Trembley 1744, who accurately describes the morphology, the extraordinary regenerative capacities and the locomotion mechanisms. Numerous species and cosmopolitan subspecies belong to the group Hydra vulgaris © Giuseppe Mazza