BRANCH CCELENTERATA : THE POLYPS, ETC. 95 



complex and is composed of different parts. Those 

 iterates which are not fixed but lead an active, free- 

 zing life, viz., the jellyfishes or medusae, are the 

 highly organized. 



ie tentacles which surround the mouth-opening and 



U* j to grasp food and carry it into the mouth, and the 



stinging or lasso threads with which these tentacles are 



provided are special organs possessed by most of these 



jials. 



^ Skeleton. Like the sponges, some of the Ccelenterata 

 possess a hard skeleton. This skeleton is always com- 

 posed of calcium carbonate and is called coral. Those 

 polyps which form such a skeleton are called the corals. 

 Coral will be described in connection with the account of 

 the coral-polyps. 



_yc Development and life-history. The polyps and jelly- 

 fishes reproduce both asexually and sexually. The 

 asexual mode is usually that of budding. On a polyp a 

 bud is formed by a hollow outgrowth of the body-wall. 

 The bud grows, an opening appears at its distal end, a 

 circlet of tentacles arises about this mouth-opening and a 

 new polyp individual is formed. This individual may 

 separate from the parent or it may remain attached to it. 

 By the development of numerous buds, and the remaining 

 attached of all of the individuals developing from these 

 buds, a colony of polyp individuals may be formed, plant- 

 like in appearance. The various polyp individuals of a 

 colony may differ somewhat among themselves, and these 

 differences are correlated with a division of labor. Thus 

 some of the individuals may devote themselves to getting 

 food for the colony, and these have mouth and tentacles. 

 Others may be devoted to the production of new indi- 

 viduals by budding or by producing germ-cells, and may 

 not have any mouth-opening or any food-grasping 

 tentacles. 



