Hydra

Hydra is a coelenterate, along with the jellyfish (Scyphozoa), sea anemones and corals and other diverse forms. Hydra

belongs to a group of coelenterates called the Hydrozoa. Hydrozoans come in a fantastic range of forms, most are quite

small, but often they form larger colonies, as in Obelia, and siphonophores. In hydrozoans there is usually an alternation of

generations as seen in the Scyphozoa, in which a tentacled polyp form, usually attached to a surface, generates sexual

medusae which are usually free-swimming bell, dome or saucer-shaped and bear tentacles. In the Scyphozoa, the

medusoid form is dominant and often large and known as a 'jellyfish'. In Hydrozoa, the medusoid is much smaller, typically 1

or 2 cm or less in diameter and more glass-like than jelly-like in appearance. The term jellyfish is sometimes also applied to

the hydrozoan medusa, especially by non-zoologists, but traditionally zoologists reserved the term 'jellyfish' for the large

scyphozoan medusae with their large quantities of jelly-like flesh (mesogloea). Hydra is different in that it has no alternation

of generations and the polyp is dominant. There is no medusoid stage in hydra, the polyp instead produces gametes when

sexually mature.

The hydra polyp is aquatic, common in fresh water that is well aerated, and quite varied in form as there are a number of

hydra species. However, all have the same basic plan - a crown of tentacles surrounds the mouth at the oral end and the

other end of the body contains an adhesive pedal disc which sticks onto solid surfaces like aquatic vegetation.</p>

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