On the face of it, when you look around, you could be forgiven for thinking that are the most succeseful form of life on this planet. Well not quite, inverts have a slight edge over us accounting for a stunning 95% of the planet's creatures. That puts us into our place then! Amazing considering that inverts are supposed to be animals with no backbones (sorry i couldn't resist). Still they probably haven't wasted as much time estimating the level of competition. Marine inverts include such creatures as Shrimps, Snails, Crabs, Anemones and Corals etc

Inverts like all other living things we have so far bothered to catalogue, are classified into groups according to their shared attributes. This system is called Scientific nomenclature. It has multiple levels of classification, each level giving a finer description, reducing the number of creatures included in it. e.g kindom could be anything furry, phylum furry with legs, class furry with legs and a pink nose and so on, all the way down to species.

The headings are:

Kingdom Phylum Class Order Family Genus Species

Corals come under the phylum Cnidaria (pronounced with a silent "c") along with jellyfish, anemones and hydra. These are all inverts with attitude. The category gets its name from specialised stinging cell-cnidae, or as they are commonly called, nematocysts. These cells, miniature chemical packed harpoons, are used both for defence of the animal and the capture of its lunch.

The animals themselves are of a fairly basic design which is radially symmetrical (the body is symmetrical around a central axis). They have a single mouth, also acting as the anus (we have a lot to be thankful for) which is surrounded by variable numbers of tentacles armed with those lovely stinging cells. This mouth opens into a single body cavity which has to do a lot of things, digesting and absorbing food being just two functions among many. This single cavity which is double walled, gives them a second name 'Coelenterata'

Cnidarians come in two distinct forms of this simple design, medusa and polyp. The Medusa version typical of jelly fish has the mouth and tentacles at the bottom, where as a polyp is the other way up with the tentacles and mouth uppermost.

Due to the lack of specialist organs (for example any form of heart) the animals are dependant on diffusion to transfer oxygen, nutrients and waste products in, out and around their bodies.

Diffusion is only efficient enough to support living tissues over a small distance. In other words any living tissue that wants to remain so, has to be within no more than 1mm of oxygen rich water. This water can be within the body cavity as well as external to the animal. Even allowing for diffusion across the body cavity wall, the available body shapes are greatly restricted. The animals either have to be very small like a coral polyp, or be made up of bits that are either thin, flat or narrow.

You may be thinking that jellyfish, for example, aren't neccessarily small, and definately aren't thin or flat. However when you take the body cavity into account, all the other structures are either thin e.g the wall of the body cavity, or long and narrow e.g tentacles.

So far we have just described the simple starting design that put the creatures into the phylum cnidaria. Adaptions to this blueprint that have given rise to hard and soft corals, anemones etc enable their further classification. To learn more please look at the individual pages I have produced on hard corals, soft corals etc.