CHAPTER 10 



The Phyla Coelenterata and 



Ctenophora 



59. Introduction 



In addition to such animals as fishes and whales which swim actively 

 through considerable distances, open water contains many organisms 

 that are passive and float aimlessly with the water currents. They may 

 swim, but not strongly enough to travel appreciably in a horizontal 

 direction or to stay in one place against a current. This assemblage of 

 organisms is the plankton, and their passive, floating way of life is called 

 planktonic. The radiohiria and foraminifera described in Chapter 8 are 

 planktonic protozoans, belonging to the marine [jhmkton. The largest 

 and most familiar of the plankton are jellyfish, often seen from ship- 

 board as vast swarms in the upper few feet of water. 



The common name, jellyfish, is applied to a heterogeneous group 

 of organisms having a jelly-like consistency, members of the phylum 

 Coelenterata and the phylum Ctenophora (Fig. 10.1). The coelenterate 

 jellyfish usually have numerous tentacles with stinging cells and swim 

 by muscular contractions of an umbrella-shaped body. The ctenophores 

 usually have two tentacles with adhesive cells, and move by the beating 

 of numerous combs, each of which is a row of fused cilia. In both phyla 

 a simple epithelium, the epidermis, covers the body, another simple 

 epithelium, the gastrodermis, lines a branched gut, and a jelly-like 

 mesoglea between the epithelia forms the bulk of the body. Both groups 

 are primarily carnivorous, catching other animals of appropriate size. 

 Small jellyfish feed upon small worms, tiny shrimplike crustaceans and 

 larval fish; larger ones catch larger fish and sometimes other jellyfish. A 

 single pelagic coelenterate is called a medusa; the ctenophore is called 

 a comb jelly. The coelenterate phylum also includes a number of 

 bottom-living forms such as hydras, sea anemones and corals, and 

 floating colonies such as the Portuguese man-of-war. A few species of 

 ctenophores creep on the bottom. As an introduction to these phyla we 

 will first describe one of the medusae. 



60. Conionemus: General Behavior 



Gonionemus (Fig. 10.2) is a genus of small medusae about 2 centime- 

 ters in diameter when fully grown. G. murbachi is a common species in 



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