Cnidarians are simple, aquatic multi-cellular animals ('metazoans') with a body made of only two main layers, technically termed a 'diploblastic' organisation. They are typically characterised by an outer ectoderm, an epithelium with muscular, sensory and stinging cells (nematocysts) and an inner digestive endoderm separated by a jelly-like mesoglea with a simple nerve net. A single opening to the body cavity or 'enteron' serves as both the mouth and anus.

The four major cnidarian groups are: the anthozoans (corals and sea anemones), scyphozoans (jellyfish), cubozoans (box-jellies) and hydrozoans (e.g. Hydra), each with distinct life cycle features and adult form. Hydroids feed and grow as sedentary, tentacular polyps from which 'medusa' larvae bud off to form a free-swimming reproductive phase, whereas the charismatic yet venomous jellyfish life cycle is dominated by the pulsating medusa or 'medusoid' phase. Corals and anemones live and reproduce exclusively via a sedentary polypoid phase. In corals each soft-bodied polyp (usually as part of a colony) usually secretes a mineralised skeleton as it grows, leaving behind chambers of hard material that can form extensive coral reef structures. Phylum Cnidaria appears to have existed since late Pre-Cambrian times (around 560 Ma) and many species are abundant in the fossil record as mineralised forms, such as Palaeozoic rugose coarls and in the Tertiary immense reefs largely constructed by scleractinian corals.

Cnidarians provide many interesting windows into convergent evolution, which you can explore through the list of assorted topics below. In highlighting just a few of these, it is remarkable to note that the highly venomous box-jellies have evolved camera eyes similar to those in much more advanced animals such as octopus and vertebrates. Cubozoan camera eyes are located on club-shaped structures (rhopalia) at the base of the bell and although their simple nervous system does not allow for complex visual processing it may be that sensory information from the eyes allows quick responses when hunting. The hanging tentacles of cubozoans and many other jellyfish species carry balancing organs termed statocysts, which house small grains (statoliths) that move and trigger nerve impulses whenever the animal changes position. Statocysts for co-ordinating balance have evolved not only in jellyfish but also independently in cephalopods and crustaceans. Deep-sea jellyfish such as Perifilia produce impressive bioluminescence as a defence strategy. Luminous light production is a convergent strategy also present in bioluminescent crustaceans (e.g. copepods), squid and even fish that inhabit the deep ocean. (In passing, it is interesting to note that terrestrial species that use bioluminescence include certain fungi and insects, for example the well-known 'fire-flies', which are actually beetles rather than flies.)

One of the most striking convergences involving cnidarians is the recruitment of photosynthetic endosymbionts called zooxanthellae. The zooxanthellae that provide 'solar power' to many corals, sea anemones (e.g. Aiptasia) and even some jellyfish are usually dinoflagellates, photosynthetic protists with whip-like flagellae. Similar endosymbioses have evolved independently in a number of non-cnidarian taxa. For example the sea slug Elysia chlorotica, acoel flatworm Symsagittifera roscoffensis and giant clam Tridacna all host photosynthetic algae, as do a variety of large protists, including foraminifera, ciliates and radiolaria. Intriguingly, polykrikid and warnowiid dinoflagellates are predatory rather than photosynthetic and have evolved a stinging-cell that is highly convergent with the cnidarian nematocyst. Perhaps most mysterious among the cnidarians are the myxozoans, an extremely derived group of parasites. Some myxozoans are convergent with parasitic protistans, and others, such as Buddenbrockia have evolved adaptations strikingly similar to the nematodes, with a long thin body that can squirm thanks to four bundles of muscles.