Cnidaria

From Palaeos

Choanomonada
`--Metazoa
   `==Porifera (paraphyletic?)
      `==Radiata (paraphyletic?)
         `--Bilateria
            `==Acoelomorpha (paraphyletic?)
               |--Protostomia
               |  |--Ecdysozoa
               |  `--Lophotrochozoa
               `--Deuterostomia

The scyphomedusae Chrysaora melanaster, the Northern Sea Nettle, (Scyphozoa,Semaeostomae,Pelagiidae,) which is common in near surface waters off the West Coast of the USA, as well as from Japan to the Bering Sea. These jellyfish have a bell approaching 30 cm across and have tentacles that can stream out from behind the animal for 3-6 meters. (Photo by Kevin Raskoff, MBARI.)

[edit] Introduction

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.

o Radiata (grade)
|?--Ctenophora
`--+--Cnidaria  
   |  |--Anthozoa
   |  `--+--Hydrozoa
   |     `--+--+--Scyphozoa
   |        |  `--Cubozoa
   |        `?-Conulata (polyphyletic?)
   `--Bilateria

[edit] Symmetry

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.

[edit] Body Form

There are two major body forms among the Cnidaria - the polyp and the medusa. The Polyp is anchored with the mouth up; the Medusa free-floating with the mouth down. 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.

[edit] Lack of Organs

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.

[edit] A mouth but no brain and no head

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) .

[edit] A Diffusion based Physiology

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).

[edit] The deadly Nematocysts

SEM of discharged nematocyst

[edit] Evolutionary History

Despite this primitive grade of organization it may be that Cnidara 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.

[edit] Classification

There are traditionally four classes of Cnidaria, which represent the three modes of lifestyle:

• Cubozoa (medusa)
• Scyphozoa (medusa)
• Hydrozoa (polyp and medusa)
• Anthozoa (polyps)

It is generally agreed that the Anthozoa are different from the others, but there has been disagreement for over a century on whether they branch first or last. The majority of recent work finds that they branched early.

[edit] Credits

Kheper MAK990603; Palaeos com MAK020616; Palaeos org MAK061028