What is Hydrozoa?
[[File:ColonialHydrozoa.jpg|thumb|250px|Close-up of a hydrozoan colony]] [[File:Portuguese Man-O-War (Physalia physalis).jpg|thumb|180px|The Portuguese Man o' War (''Physalia physalis'') long misled researchers.]] [[File:Craspedacusta sowerbyii.jpg|thumb|180px|''Caraspedacusta sowerbyii'' showing ventral side, with velum.]] The '''Hydrozoa''' are a [[Class (biology)|class]] of the [[phylum]] [[Cnidaria]]. They are small predatory animals which mostly live in the sea. Their basic life form is the [[polyp]]. Some species live singly, others live in [[Colony (biology)|colonies]]. Their classification and [[evolution]]ary relationships are still under discussion. The hydroids may be best understood by taking some examples. == Hydra == The most widely-known freshwater hydrozoan is ''[[Hydra (animal)|Hydra]]'', which is found in slow-moving waters. It is [[sessile]], with a pedal disc which attaches it to substrate. Like all cnidarians, ''Hydra'' uses [[nematocyst]]s, stinging cells which disable its prey. Hydra eat small crustaceans (such as [[brine shrimp]]), insect larvae, and [[Annelid|annelid worms]]. It may reproduce sexually, or by [[asexual reproduction]] (budding). == Colonial types == The [[Portuguese Man o' War]], ''Physalia physalis'', is a large colonial marine [[jellyfish]]. It is composed of many tiny individual [[zooid]]s, which are specialised [[polyp]]s and [[Medusa (biology)|medusae]]. The colony has an air bladder, and floats on the surface. It delivers a massive sting to its prey. Humans, if stung, may require medical attention. It is often accompanied by fish which are immune to the stings. This type of hydrozoan is called a [[siphonophore]]. The [[chondrophores]] are another type of [[pelagic]] colonial hydrozoan. They are also carnivores, but their sting is not so harmful to humans. == Freshwater jellyfish == These have a muscular ''velum'' on the ventral surface, which allows them to move about. Beginning life as a tiny [[polyp]], attached to vegetation, they feed and reproduce sexually i