23 



Coelenterates — Simple 

 Multicellular Animals 



Clusters of orange and yellow sea anemones, colonies of pink hydroids and 

 plumy sea pens well deserve their old-time names of "plant-animals" and 

 "gardens of the sea." No marine animals have such translucent beauty as the 

 coelenterates. Nor have any truly multicellular ones so long a lineage — at least 

 five hundred million years. They are direct descendants of the protozoans and 

 are the ancestors of all multicellular animals (Fig. 33.1). Sponges are also 

 directly descended from protozoans, but they long ago became set apart on 

 an offshoot of evolution. 



The two basic forms of coelenterates are the polyp and the medusa. The 

 polyp has a cylindrical body and, in its more typical condition, has one end 

 that bears the tentacles and mouth and another end attached to a surface or 

 joined to a colony. Hydra is a polyp; so are the sea anemones and corals. The 

 medusa or jellyfish has an umbrella- or bell-shaped body, is usually free- 

 swimming, and bears the sex cells (Fig. 23.1). 



Ecology. Almost all coelenterates are marine. Of about 10,000 species only 

 a few, mainly the hydras, live in fresh water. Coelenterates are widespread and 

 abundant, chiefly in surface waters and between the tide lines. Jellyfishes 

 thrive in sheltered coves rich in organic matter. They are carried about by 

 currents, great numbers of them often suddenly appearing in harbors and the 

 shallows along bathing beaches. There they feed heavily upon the swarms of 

 minute crustaceans (copepods) that become sparse soon after the jellyfishes 

 move in. Bathers know jellyfishes as sea nettles. 



Sea anemones and corals are numerous and colorful in warm seas. A few 

 inconspicuous ones occur along the more northern Atlantic and Pacific coasts 

 of the United States. Sea anemones cling tightly to rocks and wharf pilings. 

 When the tide is out, they draw their tentacles in and their bodies down almost 



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