Cnidaria
Jellyfishes, Corals, Etc. -- Cambrian to Recent
Ptilosarcus gurneyi
A close-up of the Orange Sea Pen
Class Anthozoa - Subclass
Octocorallia - Order
Pennatulacea - Suborder Subselliflorae - Family Pennatulidae
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.
Body Form
|
|
Polyp
anchored - mouth up
|
Medusa
free-floating - mouth down
|
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.
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.
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) .
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).
The deadly Nematocysts
Nematocyst with barb (top) extended
copyright © xxxx - source unknown
Nematocysts are to cnidaria what choanocysts are to sponges.
But whereas the gentle filter-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.
Evolutionary History
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 Cambrian, (or possibly
Cambrian Epoch 2), and only diversified during the Ordovician. the first unambiguous jellyfish are also not known before the Cambrian.
Classification
There are traditionally four classes of Cnidaria, which represent the three modes of lifestyle:
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, and we
follow that trend.
References and Links
Cnidaria
(Coelenterata)
Cnidaria
Cnidaria
The Cnidaria Home Page - links to material on the Cnidaria, arranged accordng to topic
Phylum Cnidaria - short intro and general information
Cnidaria - a good non-technical intro. Part of Keith Davey's Life on Australian Seashores website
Systema Naturae 2000 / Classification - Phylum Cnidaria
Cnidaria - basic outline
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page by
M. Alan Kazlev
page uploaded 16 June 2002, last modified 2 May 2003
(originally uploaded on Kheper Site 3 June 1999)
checked ATW060205
