202 ZOOLOGY 



tropical seas. Not only are many of the West Indian islands composed 

 largely of coral, but also Florida, Australia, and the islands of the southern 

 Pacific are almost entirely of coral formation. 



Coral Reefs. — The coral polyp can live only in clear sea water 

 of moderate depth. Fresh Avater, bearing mud or other impurities, kills 

 them immediately. Hence coral reefs are never found near the mouths 

 of large fresh-water rivers. They are frequently found building reefs close 

 to the shore. In such cases these reefs are called fringing reefs. The so- 

 called barrier reefs are found at greater distance (sometimes forty to fifty 

 miles) from the shore. An example is the Great Barrier Reef of Australia. 

 The typical coral island is called an atoll. It has a circular form inclosing 

 a part of the sea which may or may not be in communication with the 

 ocean outside the atoll. The atoll was perhaps at one time a reef outside 

 a small island. This island disappeared, probably by the sinking of the 

 land. The polyps, which could live in water up to about one hundred and 

 fifty feet, continued to build the reef until it arose to the surface of the 

 ocean. As the polyps could not exist for long above low water line, the 

 animals died and their skeletons became disintegrated by the action of waves 

 and air. Later birds brought a few seeds there, perhaps a cocoanut was 

 washed ashore ; thus plant life became established in the atoll and a new 

 outpost to support human life was thus established. 



Classification of Ccelenterates 



Class I. Hydrozoa. Body cavity containing no mesenteries, usually alternation 



of generation. Examples : Hydra, hydroid pennaria. 

 Class II. Scyphozoa. Examples : large jellyfishes. 

 Class III. Actinozoa. Mesenteries present in body cavity. Examples ; sea 



anemones and corals. 

 Class IV. Ctenophora. 



Reference Books 

 for the pupil 



Herrick, Text-book in General Zoology, Chap. VII. American Book Company. 

 Davison, Practical Zoology, pages 167-175. American Book Company. 

 Holder, Half Hours with the Lower Animals. American Book Company. 



FOR THE TEACHER 



Agassiz, A First Lesson in Natural History. D. C. Heath and Company. 

 Dana, Coral and Coral Islands. Dodd, Mead, and Company. 

 Parker, Elementary Biology. The Macmillan Company. 



