CCELENTERATA 



189 



have changed the land contours by additions of marginal reefs. 

 Much of the limestone of the crust of the earth is from corals. 



Pig. 87. 



Fig. 87, A very diagrammatic and generalized illustration of a complex coelenterste colony. 

 The shaded portion represents the gastro-vascular cavity. The light portion, the body tissues, 

 b., a bell-like individual developed into an air-bladder; m., mouth; n.z., nutritive individual; p.t., 

 protective individual; r.».>, r.tJ, r. «.», different types of reproductive individuals; s.b., swimming 

 bell; (, tentacles, which are sensory and protective structures. After Lang. 



Questions on the figure. — What is meant by "generalized" above? How does 

 such a polymorphic colony as this differ from a highly organized individual? 

 In what respect is it similar to an individual? What is the function of the gastro- 

 vascular system? What is the gain in its wide distribution through the colony? 

 How do the Siphonophora differ from the other colonial coelenterates? 



