Cnidaria and Ctenophora

Cnidaria

General

Hydrozoa

Anthozoa

Scyphozoa

Ctenophora

CNIDARIA

The Cnidaria (sometimes called the Coelenterata) are comprised of a variety of aquatic organisms including the corals, sea anemones, jellyfish, corals, and sea fans. Most of the species are marine, but a few are fresh water such as the hydra so much beloved of school biology laboratories. Although the Cnidaria have a basically simple, radially symmetric body plan despite superficial differences in the animals, there is more cell specialization than in the sponges. The outer epidermis contains sensory-nerve cells, gland cells, stinging cells called nematocysts, and contracting cells called epitheliomuscular cells. The body usually consists of a double layer of cells (epidermis and gastrodermis outside and inside respectively) with a separating, non-cellular, gelatinous layer called the mesoglea. This double layer of cells encloses a cavity called the gastrovascular cavity opening to the outside by a mouth. The life cycle of a typical cnidarian consists of two different stages called a medusa and a polyp. The sea anenome and the hydra are examples of the polyps while a jellyfish is a perfect example of a medsua. Although the two stages look quite different, in structure the medusa is an "upside down" free living form of the polyp. Many Cnidaria have both a polyp and a medusa form during their life cycle. In the corals and sea anenomes, however, the medusa (free living stage) has been lost.

The Cnidaria are divided into three major groups.

Hydrozoa - The Hydrozoa consists of the hydra-like animals. Both medusa and hydra stages exist in this species, although the hydra (polyp) stage is by far the more conspicuous.

Scyphozoa - The Scyphozoa are the jellyfish. The polyp stage is very small and inconspicuous and the medusa (jellyfish) stage dominates.

Anthozoa - The Anthozoa consists of the corals, sea anenomes, sea fans, and a variety of other marine organisms. The medusa stage of the life cycle has been completely lost in these animals.

CTENOPHORA

The Ctenophora (comb jellies or sea walnuts) look very much like jelly fish and share many of their morphological features such as radially symmetry, a sack-like body with a wall consisting of three layers. Unlike the Cnidaria, however, they lack independent mesodermal muscles, nematocysts (stinging cells), and the polyp-medusa life cycle. The group is characterized by eight ciliary plates (combs). The cilia in these combs wave in unison and enable the animal to swim about, although somewhat weakly.

GENERAL

Introduction to the Cnidaria. Museum of Paleontology, University of California at Berkeley. An excellent introduction to the Cnidaria including information on biology, morphology, systematics and fossil record, life history, and ecology.

Specimen database. Marine Biology Laboratory, Woods Hole. Photographs and information on species of marine Cnidaria found in the Woods Hole region of Massachusetts.

Cnidaria. Tree of Life. Morphology and systematic relationships among the Cnidaria with a phylogenetic tree.

Cnidarians. An introduction to the Cnidaria with some photographs.

The Cnidaria Homepage. Rob Steele, University of California at Irvine. Links to webpages on Cnidarians.

HYDROZOA (Hydras)

The Hydrozoa Directory. Peter Schuchert, Museum de Geneve. A wonderful collection of general and specific information as well as photographs displaying the diversity of the Hydrozoa.

ANTHOZOA (Corals, Sea Anenomes, Sea Fans)

A Key to Some Common Shallow Water Caribbean Reef Corals. A great page with photographs, information, and identification keys to the corals of the Caribbean. Although these species are Caribbean, many of the species will also be found in the coral reefs of southern Florida.

Anthozoa. Keys to Marine Invertebrates of the Woods Hole Region. Marine Biological Laboratory. Keys to the species of marine Anthozoa in the Woods Hole region of Massachusetts.

Tropical Marine Ecology of the Florida Keys, Everglades, and Bahamas. R.H. Cummins. Photographs of corals and coral reefs.

Anthozoa.com. A great collection of information and photographs of corals and their relatives.

Cnidarians, Anthozoa. Smithsonian National Zoological Park. A short, but good introduction to the corals and other anthozoans.

Ocean Oasis Field Guide. San Diego Museum of Natural History. A good, general introdution to the anthozoa including the corals.

SCYPHOZOA (Jellyfish)

Scyphozoa. Keys to Marine Invertebrates of the Woods Hole Region. Marine Biological Laboratory. Keys to the species of marine Scyphozoa in the Woods Hole region of Massachusetts.

Jellies Zone. David Wrobel. A wonderful web site about all of the animals in the seas that are gelatinous. Included are the jellyfish and ctenophores. For each group you'll find general information on the group as well as photographs and information about specific species, particularly from the west coast of North America. Highly recommended.

Scyphozoa. Keys to Marine Invertebrates. Woods Hole Marine Biological Laboratory, Dale Calder. An introduction to the jellyfish and a key to the species from northeastern North America.

Introduction to the Ctenophora. Museum of Paleontology, University of California at Berkeley. Basic information on the Ctenophora (comb jellies) with a photograph.

Ctenophora. Key to Marine Invertebrates of the Woods Hole Region. Marine Biological Laboratory. A key to the Ctenophora of the Woods Hole region of Massachusetts with an annotated list of species.

Jellies Zone. David Wrobel. A wonderful web site about all of the animals in the seas that are gelatinous. Included are the jellyfish and ctenophores. For each group you'll find general information on the group as well as photographs and information about specific species, particularly from the west coast of North America. Highly recommended.

Comb Jellies. Microscopy UK, Wim van Egmond, A concise, but excellent introduction to the comb jellies (Ctenophora).

Ctenophora Specimen Database. Marine Biological Laboratory, Woods Hole. Photographs and some information on two species of Ctenophora found in the Woods Hole region of Massachusetts.

Phylum Ctenophora: List of all Valid Scientific Names. Caludia E. Mills, University of Washington. A synonymical checklist of the ctenophore species of the world with links to photographs of several of the species.

Ctenophora. Tree of Life. Classification of the Ctenophora with a nice photograph.

Pictures of Plankton. Steven Haddock. A gallery of photographs of several species of ctenophores.