Phylum Cnidaria

This diverse group of very simple-bodied animals includes corals, sea anemones, hydras, jellyfishes, and their relatives. About 9,000 living species are known. The Cnidaria are the simplest Metazoa, and do not even possess organs. All they have is a stomach and a mouth surrounded by tentacles.

All cnidarians are radially symmetrical (the body is symmetrical around a central axis). As with the plant, there is no front or rear, but there is a top and bottom.

There are two major body forms among the Cnidaria - the polyp and the medusa. Sea anemones and corals are typical of the polyp form, while jellyfish are typical medusae When you look at it, you can see that these are the same body form, except that one is upside down, while the other is not.

Cnidaria have no organs (i.e. groupings of different tissues to perform complex functions e.g. hearts, lungs, kidneys, etc). There is a gastrovascular cavity ("stomach" in the broad sense) with a mouth but no anus, a body wall with outer epidermis and inner gastrodermis, between which is a jelly-like mesogloea.

The nervous system is arranged as a decentralized network (‘nerve net’), with one or two nets present. There is no brain or ganglia.

Note that although there is no head, there is a mouth, surrounded by a crown of tentacles. The tentacles are armed with deadly nematocysts (stinging cells) .

All the metabolic functions of the body - respiration, digestion, elimination - are carried out by diffusion. Diffusion is only an efficient means of exchange of materials only over short distances (e.g. over about 1 mm for oxygen exchange). This means that all the tissues of a cnidarian which require oxygen (all its living cells) must be within about 1 mm of a surface which is bathed by oxygenated water. This is why cnidarians are either very tiny (e.g. coral polyps) or have very thin or flat bodies or long thin tentacles (both of which increase surface area). And also because of this diffusion based physiology, cnidarians can only live in water (most species are marine, but some Hydrozoa live in fresh water). There are only two surfaces in a cnidarian through which this diffusion can occur - the epidermis (the outer layer or skin, which is bathed in water at all times) and the gastrodermis (the inner layer, representing the stomach lining).

Nematocysts are to cnidaria what choanocysts are to sponges. But whereas the gentle filtler-feeding sponge simply sweeps microscopic food particles to it, the predatory cnidarian uses batteries of deadly stinging cells that can paralyze and kill quite large prey. These large cells have a sealed central cavity of poisonous fluid. A small sensory hair is sensitive to vibrations in the water. Any luckless creature passing too close triggers the nematocyst which then shoots out the barb, which penetrates the prey so it can be injected with poison. The prey is then conveyed to the mouth by the tentacle.

Despite this primitive grade of organization they are no more ancient than any other group of animal. The Edicarian biota, supposed jellyfish and soft coral (sea pen) fossils from the latest Proterozoic (Ediacaran) era have turned out to belong to a totally different type of organisms, the enigmatic "Vendobionta". although there is still some argument for diploblastic natuer of these organisms. In any case it is inconceivable that large predatory organisms like jellyfish could have existed at a time when there was nothing else around for them to feed on! The first coral organisms only appear in the middle, (or possibly Early) Cambrian and only diversified during the Ordovician. the first unambiguous jellyfish are also not known before the Cambrian.

Links

The Cnidaria Home Page - links to material on the Cnidaria, arranged accordng to topic

page uploaded 16 June 2002, last modified 19 November
(originally uploaded on Kheper Site 3 June 1999)
checked ATW020814
page by M. Alan Kazlev
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unless copyright otherwise indicated. this material may be freely used for non-commercial purposes

Phylum Cnidaria

(formerly Coelenterata)

Cambrian to Recent

The Cnidaria

Symmetry

Body Form

Lack of Organs

A mouth but no brain and no head

A Diffusion based Physiology

The deadly Nematocysts

Evolutionary History

Classification

Links


Polyp anchored - mouth up	Medusa free-floating - mouth down