210 THE METAZOA — DIVISION OF LABOR 



polyps, each oonneeted with its neighbor. In this way great masses of 

 coral are formed. Coral, in a living state, is alive only on the surface, 

 the polyps building outward on the skeleton formed by their predecessors. 



Economic Importance of Corals. — Only one (astrangia) of a great 

 many different species of coral lives as far north as New York. In tropical 

 waters they are very abundant. Coral building has had and stiU has an 

 immense influence on the formation of islands, and even parts of conti- 

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

 composed largely of coral, but also Florida, Austraha, and the islands of 

 the southern Pacific are almost entirely of coral formation. 



Coral Reefs. — The coral polyp can hve' only in clear sea water of 

 moderate depth. Fresh water, bearing mud or other impurities, MUs 

 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-caUed 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 communica- 

 tion 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 coconut was washed ashore ; thus plant life became estab- 

 lished in the atoll, and a new outpost to support human life was estab- 

 lished. 



Classification of Ccelenterates 



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

 of generation. Examples : Hydra, hydroids. 



Class II. Scyphozoa. Examples: large jellyfishes. 



Class III. Actinozoa. Mesenteries present in body cavity. Examples : sea anem- 

 ones and corals. 



Class IV. Ctenophora. 



Reference Books 

 elementary 



Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American 



Book Company. 

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

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

 Jordan, Kellogg, and Heath, Animal Studies. D. Appleton and Company. 



