210 



PRACTICAL ZOOLOGY 



According to Darwin, the reef was originally built up around an 

 oceanic island which slowly sank beneath the ocean, leaving the 

 coral reef inclosing a lagoon. 



Besides producing islands and reefs, corals play an important 

 role in protecting the shore from being worn down by the waves. 

 They have also built up thick strata of the earth's crust. 



FIG. 126. Whitsunday Island in the South Pacific, an atoll built by corals. 

 (After Darwin.) 



Characteristics and Classification. The ccelenterates are 

 aquatic animals, mostly marine. They are radially symmetri- 

 cal, have a single gastrovascular cavity, and are provided with 

 stinging cells, the nematocysts. 



The three classes of ccelenterates are as follows: 



Class i. HYDROZOA. Fresh-water Polyps, Hydroid Zo- 

 ophytes, many small Medusae or Jellyfishes, and a few stony 

 Corals. 



Class 2. SCYPHOZOA. Most of the large Jellyfishes. 



Class 3. ANTHOZOA. Sea Anemones, most stony Corals, 

 Sea Fans, Sea Pens, and precious Corals. 



REFERENCES 



Cambridge Natural History, Vol. I. The Macmillan Co., N. Y. City. 

 College Zoology, by R. W. Hegner. The Macmillan Co., N. Y. City. 

 The Structure and Distribution of Coral Reefs, by Charles Darwin. D. 

 Appleton & Co., N. Y. City. 



