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Phylum Coelenterata 



(Cnidaria). Simple 



Tissue Animals 



102 



HE phylum Coelenterata contains a large 

 number of interesting animals, but most of 

 the 10,000 or more species live in salt water 

 and are seen alive by only a small proportion 

 of those interested in animal life. However, 

 they exhibit the characteristics of the lower 

 Metazoa to good advantage; and one type, 

 the hydra, common in fresh water, affords 

 excellent material for laboratory study. Cer- 

 tain types that live in the sea and that serve 

 well as examples of the larger division of the 

 phylum are also described briefly and illus- 

 trated; these include the colonial hydroid 

 Obelia, the hydrozoan jellyfish Gonionemus, 

 the scyphozoan jellyfish Aurellia* the sea 

 anemone Metridium, and the coral Astran- 

 gia. The sponges do not exhibit well-devel- 

 oped tissues, but the coelenterates have 

 reached a definite tissue-level of organiza- 

 tion. Another advance in their level of or- 

 ganization over the sponges is that the cells 

 are more highly specialized and integrated; 

 they are more receptive to stimuli and are 

 capable of a great variety of responses. 



The coelenterates may be used to illus- 

 trate many important biological phenomena 

 such as budding, certain types of behavior, 

 regeneration, grafting, colony formation, 

 metagenesis, and polymorphism. These are 

 all exhibited by the Hydrozoa, hence it is 

 suggested that this class be studied more 

 thoroughly than the Scyphozoa and Antho- 

 zoa. 



Coelenterates are radially symmetrical 

 animals. The principal axis extends from 

 the mouth to the base; similar parts are 

 arranged around this axis in a circle. The 

 body wall consists of two layers of cells, 

 between which is a noncellular substance, 

 the mesoglea (Fig. 58). Within the body is 

 a single gastrovascular cavity (Fig. 51). The 

 coelenterates are provided with stinging 

 capsules called nematocysts (Fig. 54). 



The phylum contains three classes: (1) 

 the Hydrozoa, including the fresh-water 

 polyps, the small jellyfishes, the hydroid 



* Usually incorrectly spelled Amelia. The original 

 and hence correct spelling is Aurellia. 



