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Bell Pettigrew Museum
of Natural History
Interpretative Panels
Text: Dr Iain Matthews
Design: Steve Smart & Cavan Convery
A University of St Andrews Development Fund Project
School of Biology
http://biology.st-andrews.ac.uk
3:2 Cnidaria
P hy l u m
Body Plan:
• Radially symmetrical
with tentacles
• Diploblastic
• No enclosed body cavity
• Central ‘gut’ space with
mouth, no anus
Cnidaria
S u b p hy l u m
Super class
Class
The phylum Cnidaria is a large and diverse group
of radially symmetrical animals that, despite their
simple body form, can achieve high levels of
complexity. They have a long fossil record, stretching
back over 700 million years, however they have never
moved onto land and all members of the phylum
are aquatic. The majority are marine, although there
are a small number of freshwater species.
Their basic tissue level organisation is not radically
different from that of sponges; two layers of cells lie
on either side of the gelatinous mesogloea, forming
a tube that encloses a central coelenteron, which has
only a single opening surrounded by tentacles. The
tentacles are covered with specialised stinging cell
organelles, the nematocysts, and are used for both
offence and defence. When triggered, threads are
fired out from the nematocyst and inject a toxin into
the prey, which are then drawn back in for digestion.
Gut
Endoderm
Ectoderm
secretion of an external casing, the calyx. Those with
a calyx of calcium, the corals, have created vast reefs
of great ecological and geological significance.
However the calyx is merely a hard casing and the
true skeletal system of the cnidarians is the mesogloea
and/or the water contained within the coelenteron.
The cnidarians show marked structural and life
style diversity. Forms include solitary polyps (e.g.
anemones and hydras), colonial sessile polyps (e.g.
corals
and hydroids ), colonial floating forms
(e.g. siphonophores
) and solitary floating medusae
(e.g. jellyfish ).
Classification
within
Cnidaria
Subphylum: Medusozoa
Cnidarians exist in two distinct body forms; either
as polyps, such as anemones and corals, or medusae
such as jellyfish. On this basis, two subphyla are
classified: the Medusozoa, in which a medusa occurs
in the life history and the Anthozoa in which a
medusa stage is lacking.
Some cnidarians, through repeated asexual
reproduction, form large colonies often with the
See specimen.
Subphlyum: Anthozoa
Class: Hydrozoa
Order: Limnomedusae
Order: Laingiomedusae
Order: Narcomedusae
Order: Trachymedusae
Order: Actinulida
Order: Anthoathecata
Order: Leptothecata
Order: Siphonophora
Class: Alcyonaria
Order: Protalcyonaria
Order: Stolonifera
Order: Telstacea
Order: Gastraxonacea
Order: Gorgonacea
Order: Coenothecalia
Order: Alcyonacea
Order: Pennatulacea
Class: Scyphozoa
Order: Stauromedusae
Order: Coronatae
Order: Semaeostomeae
Order: Rhizostomeae
Class: Zoanatharia
Order: Actiniaria
Order: Zoanthinaria
Order: Scleractinia
Order: Corallimorpharia
Order: Ptychodactiaria
Order: Antipatharia
Order: Ceriantharia
Class: Cubozoa
Order: Cubomedusae
Jellyfish, anemones corals and hydroids
Some anemones
have a special relationship with
certain fish (mostly clownfishes), which live in
amongst the tentacles and secrete a special mucus
to avoid being stung.
Some scyphozoan jellyfish (e.g. the lion’s mane
jellyfish, Cyanea capillata
) can reach over 2
metres in diameter and have tentacles 60 to 70
metres long.
The sea wasp is one of the world’s most dangerous
jellyfish. It kills several people each year, most
commonly from the tropical waters off Australia.
Witnesses report victims covered in metres of
tentacles and if the sting is sufficient death occurs
in about 15 minutes.
The cnidarian corals are responsible for building
the huge reefs such as Australia’s Great Barrier Reef.