From Simple English Wikipedia, the free encyclopedia
Close-up of a hydrozoan colonyThe Portuguese Man o' War (Physalia physalis) long misled researchers.Caraspedacusta sowerbyii showing ventral side, with velum.
TheHydrozoa are aclass of thephylumCnidaria. They are small predatory animals which mostly live in the sea. Their basic life form is thepolyp. Some species live singly, others live incolonies. Their classification andevolutionary relationships are still under discussion. The hydroids may be best understood by taking some examples.
The most widely-known freshwater hydrozoan isHydra, which is found in slow-moving waters.
It issessile, with a pedal disc which attaches it to substrate. Like all cnidarians,Hydra usesnematocysts, stinging cells which disable its prey. Hydra eat small crustaceans (such asbrine shrimp), insect larvae, andannelid worms. It may reproduce sexually, or byasexual reproduction (budding).
ThePortuguese Man o' War,Physalia physalis, is a large colonial marinejellyfish. It is composed of many tiny individualzooids, which are specialisedpolyps andmedusae. 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 asiphonophore.
Thechondrophores are another type ofpelagic colonial hydrozoan. They are also carnivores, but their sting is not so harmful to humans.
These have a muscularvelum on the ventral surface, which allows them to move about. Beginning life as a tinypolyp, attached to vegetation, they feed and reproduce sexually in the spring and summer. If the habitat has only one sex, they reproduce by budding. They overwinter as resting bodies.
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