Polyps and medusae

Two layered
Cross section of a cnidarian.

There are two basic forms of cnidarians: free swimming medusae and attached polyps. Certain cnidarians can change between these forms, while others are either medusae or polyps all their life.
    Cnidarians can be found in varying sizes, from a microscopic polyp, to the largest jellyfish, that can be 2 metres over the "umbrella" and with tentacles over 30 metres long. They can live a solitary existence or in colonies with many other individuals. Coral reefs are composed of a great many cnidarian colonies and can cover large areas of the sea-floor.
    Cnidarians are found only in a Marine environment, some of which in freshwater, but mostly in seawater.

Two layers

The anatomy of the cnidarian is of a very simple nature. It consists of two layers, an outer (ectoderm) and an inner (endoderm). The inner layer is basically built like a sac and of the same form as the outer. It is within the walls of these two layers that the stinging cells, muscles and nerves are found aswell as where digestion takes place. Between these two cell layers is a layer of jelly or fibre (mesoglea), that contains hardly any cells. The mesoglea gives the animal its form. The mouth of the cnidarian is situated in the middle of its round body. It is here food is inserted and the undigested remains are ejected. The stomach can be very branchy and this can aid the transport of nourishment within the animal. The tentacles are commonly found around the mouth.

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Bo Johannesson | Martin Larsvik | Lars-Ove Loo | Helena Samuelsson