254 Skeletons op the lower ajstimals 



In certain of the coelenterates, such as the coral polyps, 

 the walls secrete a limestone matter, which increases to 

 such amounts that the animals which produce it finally 

 lie in crevices of this hardened secretion, which thus forms 

 masses often larger than the animals themselves. 



These secreted skeletons are all more or less rigid, and 

 while they function as supports, or protective material, they 



Fig. 103 — A coral colony ; living polyps in limestone which they have secreted. 



are entirely without joints. Such a skeleton is evidently 



entirely unadapted for animals that are to move about in 



search of food, and whose parts 



must be capable of movement. 



Such animals require a modifica- 



flij ^^ tion of this rigid, one-piece skele- 



FiG. 104 — Arrangement of ton, and in the starfish we find 



plates in a starfish ; M, soft > -, i .,,.,. 



membrane ;P, hard plates. ^ Simple change JU thlS direction. 



These animals are covered with 

 an external limestone secretion which is in the form of 

 plates, but these plates are small and are united by con- 



