HYDR^, JELLYFISHES, SEA ANEMONES, CORAL POLYPS 57 



Coral polyps. — Most of the sea anemones are simple, 

 i.e. each annual is separate, living by itself, although it 

 may reproduce by budding. On the other hand the coral 

 polyps, although they are very similar to the sea anemones 

 in structure, are usually compound, and live in colonies. 

 These are the animals that mainly build up the great coral 

 reefs, found so abundantly in the Pacific Ocean. The 

 structure of each little animal, or polyp, as it is called, is so 

 similar to that of the sea anemone that it needs no further 



Fig. 26. — Corals : A, tree coral ; B, tree coral ; C, head coral. 



description here. A coral polyp differs from a sea anemone 

 greatly in one particular, however; viz. in the power to 

 secrete lime carbonate. This mineral substance is secreted 

 by the ectoderm of the lower part of the polyp. In a large 

 colony of polyps in which new individuals are constantly 

 appearing, and in which each individual is adding its share 

 of lime carbonate to the secretions of the others, there is 

 finally built up in the process of time a great mass of lime 

 carbonate. The shape of this mass will depend upon the 

 species of polyp building it. 



