Cnidaria include the anemones, corals, jellyfish and hydroids, i.e. all the animals formerly included with the comb jellies (‘Ctenophora’) in the phylum Coelenterata. They may be in the form of sessile polyps, or freely floating medusae (Figure 4.1a,b). All are aquatic, nearly all are marine, and they are very simple in structure. Yet there are vast numbers of individuals belonging to at least 10 000 species widely dispersed in the sea, varying in size from individuals a few millimetres across to coral colonies measuring hundreds of metres. Stinging cells (called ‘cnidae’ or ‘nematocysts’) are used for food capture and defence; they are unique to the phylum and diagnostic of it.

The combination of simplicity of structure with large numbers and considerable diversity provides the theme for this introduction to the phylum. Discussion is focused on how such simple animals can make a living and what features have enabled them to become so diverse. This chapter is relatively rather full, because the emerging picture forms an important background to the consideration of more elaborate animals.

Why do we regard Cnidaria as simple?

They have no head end. The mouth (which serves also as the anus) is the single opening of the only internal cavity, called the ‘coelenteron’, which is an enclosed part of the water in which the animal lives. The mouth is usually surrounded by tentacles where the stinging cells are concentrated.