C(ELENTERATA. 183 



animals are more successful here. The long slender 

 tentacles, sensitive to touch and continually waving about, 

 are the organs by which the food is detected and caught. 



The jelly-fish are used as food by fishes and whales, but 

 they do not contain much nourishment since only about 

 i per cent, of their body is solid matter. The other 

 species do not seem to furnish much food for other animals. 

 The red coral of the Mediterranean is used in making 

 jewelry; this species does not secrete its skeleton as rapidly 

 as the more common species. 



Corals have added considerable land to the margins of 

 continents by forming the fringing reefs a few miles from 

 shore, as along the coast of Florida. The washings 

 from the land finally fill up the space between, the plants 

 begin to take root, and gradually a new body of marshy 

 land fringes the old land. A large part of the peninsula 

 of Florida has a coral foundation. This is true of many 

 of the islands of the tropical .seas. 



201. Definition of the Coelenterata. The most marked char- 

 acteristics of the group are: the single (gastro- vascular) cavity, 

 lined with entoderm, which has only one opening (the mouth) ; 

 there is no separate "body cavity" (see 122) as in the higher animals; 

 they have their external organs (tentacles) and many of the internal 

 structures arranged radially about the long axis, at the free end of 

 which the mouth lies; the nettling cells are the protective structures. 



202. Key to the Glasses of Coelenterates. 

 True nettling cells. 



Mouth opening directly into gastro-vascular cavity. 



Class I Hydrozoa. 

 Polyps attached; medusae, when present, small. 



Order Hydromedusce. 

 No medusae: example: Hydra. 



Attached medusa-like bodies; examples: Cordylophora, 

 Plumularia. 



Medusae escape and swim freely; examples: Obelia, Bou- 

 gainville a. 



