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Hydrozoans

(Class: Hydrozoa)

A diverse group of cnidarians (jellyfish, corals, hydras, sea anemones, sea ferns and related organisms), that inhabit both marine and freshwater environments, and can be colonial (mostly) or solitary in nature. They are mostly characterised by a life cycle that contains a planula larval stage (which normally develops into a sessile polyp) and the medusa stage, a free-swimming breeding stage. They all contain stinging cells called nematocysts (or cnida), which are used to capture prey or repel predators, and some can cause severe reactions in humans. The Portuguese man o’wars are a familiar sight in Australian waters and washed up on beaches. Although it is regarded by many to be a single multicellular organism, it is actually a colony of small, specialised organisms called zooids that live together and perform specialised functions. Often referred to as a bluebottle, due to its colouration, the larger portuguese man o’war has been responsible for human fatalities.

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