FRESH-WATER POLYP AND SOME ALLIES 271 



Hydroids and Medusae. There is a large group of animals 

 closely allied to the hydra, that live in the sea. A portion 

 of one of them, Bougainvil'lia 

 frutico'sa, is shown in Fig. 142. 

 It looks very much like a plant, 

 owing to its habit of branching. 

 The colony may contain many 

 polyps, each of which is con- 

 nected indirectly with every 

 other by means of the continu- 

 ous body wall. 



There are two different kinds 

 of individuals in such a colony. 

 One of these very closely re- 

 sembles the Hydra. The other 

 is a minute jellyfish, or medusa, 

 which is formed by the colony, 

 but later separates from the col- 

 ony and leads an independent, 

 free-swimming existence. At the 

 base of the colony, root-like 

 branches from a stem-like struc- 

 ture cling to floating timbers or 

 buoys. Although the colony 

 reaches the height of two inches, 

 the individual polyps are micro- 

 scopic in size. Each polyp has 

 about fourteen tentacles amply 

 provided with nettling capsules, 

 which aid in capturing small 

 pelagic animals. The mouth is 

 at the center of the circle of ten- 

 tacles (Fig. 142, 1) ; it opens directly into the gastrovascular 

 cavity. As the branches are terminated by feeding polyps, 



Fig. 142. Hydroid. (Much 

 enlarged) 



1, tentacles; 2, 3, k, stages in the 



formation of medusa. (After 



Allman) 



