Anthozoa
Scyphozoa
Cubozoa
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The phylum Cnidaria ("ny-dare-ee-ah")
includes an odd assortment of simple animals: sea anemones and jellyfishes.
There is tissue level organization in these animals, but the only true organs
are the gonads. They possess a nerve net that controls body activity, but there
is no brain or centralized nervous control. The body is radially symmetrical. All species are aquatic or marine,
though many, such as the anemones, can withstand many hours exposed to the air
during low tides.
The phylum gets its name from the
cnidocytes,
specialized cells that contain hypodermic stinging structures attached to
filaments, and small venom sacs. When these cells are in contact with prey, they
fire the tiny hypodermic harpoon (called a nematocyst) and its long tube connecting
with the venom sac. The microscopic size of the cnidocytes means that dozens to
hundreds are typically fired simultaneously. The tubes keep prey attached to the
tentacle until the venom induces paralysis. These microscopic dart-and-line
structures are what give tentacles of jellyfishes and anemones their stick
feeling when touched. Most cnidarians are harmless to humans. Some, such as the
largest of them all, the lion's mane jellyfish, may cause severe pain. The Portuguese
man-o'-war causes excruciating pain, but no human deaths can be attributed to
the venom of this species. At the other extreme is the Australian sea wasp, the
venom of which may cause death within minutes.
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At left is a hydrozoan
jellyfish. This Pacific resident possesses numerous stinging tentacles and
a reddish luminescent region within the rim where the tentacles attach to
the bell. This species grows to a total length of some 5 inches across the
bell; the tentacles, when relaxed, may extend over 8 feet. The fresh water
hydras, so often studied in biology classrooms, are probably the best
known members of the class Hydrozoa. Photo by Dr. R. G. Sprackland. |
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The phylum contains four classes: the Hydrozoa (meaning
"water animals"), which includes many jellyfishes, including several
fresh-water species, and the hydras, which generally possess a velum; the
Scyphozoa ("cup animals"), which includes the stinging sea nettle
jellyfish; the Anthozoa ("flower animals"), which are the least like
jellyfish in the phylum, including anemones, corals, and sea fans; and the
Cubozoa ("box animals"), including box jellies and the most venomous
animal known, the sea wasp jellyfish.
Jellyfishes (which are neither fishes nor closely
related to vertebrates) have an unusual ability to shrink in size when food
supplies are lacking, and growing again when food is plentiful.
Characteristics of Cnidaria
include:
 | Radial body symmetry, |
| Two tissue levels (not
three)--endoderm and ectoderm only, |
| A jelly like mesogleal layer
between the endo- and ectodermal layers, |
| Cells with joint nutritive
and muscular functions, |
| Epithelial muscular cells, |
| Cnidocytes with
venom-conducting nematocysts, |
| Lack of any true organs
excepting gonads, |
| Gastrovascular cavity present |
Literature: Click on
a book to order a copy.
Mitchell,
L., J. Mutchmor and W. Dolphin. 1988. Zoology. Benjamin/Cummings
Publishing, Menlo Park, CA. ISBN: 0-8053-2562-X.
Rees,
W.J. (editor). 1966. The Cnidaria and their Evolution. Symposia of the
Zoological Society of London, No. 16. Academic Press, NY.
Sprackland, Robert. 1994. Jellyfish primer for
the marine aquarist. Tropical Fish Hobbyist 42(9): 32-39.
White, Julian. 1995. CSL Antivenom Handbook.
Commonwealth Serum Laboratories, Melbourne. ISBN: 0-646-26814-7.
Link:
Cnidaria WWW Server
(jellyfishes & corals): www.ucihs.uci.edu/biochem/steele/default.html
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