634 - Heredity and Evolution 





Fig. 32-10. Skeletal supporting and attaching structures of various coral animals. Coral reefs are built mainly 

 of such structures, which may accumulate enormously through the years. (Courtesy of Ward's Natural Science 

 Establishment.) 



nant role in the building of coral reefs. 

 Undoubtedly the greatest of all coral reefs 

 is the Great Barrier Reef, which extends more 

 than a thousand miles along the northeast 

 coast of Australia, at varying distances up to 

 about 80 miles offshore. However, smaller 

 coral reefs are very common in South At- 

 lantic and South Pacific regions. Layer after 

 layer of calcareous deposits, representing 

 countless generations of living animals, tend 

 to accumulate in the warm shallow offshore 

 waters, at a rate (measured under present-day 

 conditions) varying from 5 to 200 millimeters 

 annually (see Fig. 32-1 1). 



Phylogeny of the Coelenterata. Coelen- 

 terate fossils extend back into formations of 

 early Cambrian period — which demonstrates 

 a very ancient origin. Also the embryological 

 pattern and other evidence indicate that 

 some of the early Coelenterata represent a 

 main line in the evolutionary ascent of all 

 major groups of higher animals (Fig. 29-11). 



THE CTENOPHORA (LITERALLY, COMB 

 BEARERS) 



The animals in this small group (only SO 

 species) of exclusively marine forms resemble 



rocky oceanic islet 



encircled by 

 fringing coral reef 



reef enlarges 



as land sinks 



(or sea level rises) 



circular 

 coral reef around 1 

 an atoll (with further 

 change in level) 



Fig. 32-11. Darwin's concept of how coral reefs are formed. (By permission from General Biology, by Tracy I. 

 Storer. McGraw-Hill Book Co., Inc.) 



