The Portuguese Man o' War (Physalia physalis) long misled researchers.
Caraspedacusta sowerbyii showing ventral side, with velum.
The Hydrozoa are a class of the phylumCnidaria. 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 colonies. Their classification and evolutionary relationships are still under discussion. The hydroids may be best understood by taking some examples.