Coelenterata. 329 



known, they cannot build coral in cold water, being 

 limited to about the temperature of 60 F. The other 

 group have the tentacles on the plan of eight, and are 

 sometimes therefore called Octocoralla. They make little 

 coral, but form whiplike and fanlike colonies, the sea 



FIG. 187. RED CORAL POLYPS. 



fans, and sea pens, or sea whips. These have a horny 

 core with loose limy material on the outside. To this 

 group belongs the well-known red coral of the Mediterra- 

 nean. The polyps of many of the corals are exceedingly 

 brilliant. 



GENERAL CHARACTERISTICS OF CCELENTERATA. 



1. There is no digestive tube separate from the body 

 cavity. 



2. Stinging cells are almost always present. 



3. The body is usually radially symmetrical, but in some 

 there are traces of bilateral symmetry. 



