FRESH-WATER POLYZOA. 225 



begins the colony; it becomes two by a process of bud- 

 ding, the bud finally becoming another Polyzoon, se- 

 creting more jelly, budding in its turn, so that the com- 

 munity may in the end contain numberless members, 

 and the mass may measure several feet in diameter. 

 The color of the animals is usually a pale red or flesh 

 tint, deepening to crimson about the mouth, which is 

 placed near the centre of the crescentic or horseshoe- 

 shaped disk of tentacles. In the largest, and therefore 

 the oldest, colonies the jelly may exhibit many scat- 

 tered white spots composed of carbonate of lime. 



There is another jelly-forming colony called Crista- 

 tella, which the beginner may mistake for young Pecti- 

 natella. It is to be distinguished by the absence of 

 those great masses which characterize Pectinatella, by 

 the general appearance of the colony, and by its motion. 

 A community of Cristatella is usually long and narrow, 

 often measuring several inches in length. One species 

 is about eight inches long, one-fourth of an inch wide, 

 and one-eighth thick. Young colonies are, of course, 

 smaller, and are rounded. It has the power which no 

 other fresh-water Polyzoon possesses to travel from 

 place to place. It moves very slowly, a colony about 

 an inch in length moving an inch in twenty-four hours. 



All the fresh-water Polyzoa, of which there are sev- 

 eral genera and species, have on the front part of the 

 body a disk which bears the tentacles. It is named the 

 lophophore, and is, in some forms, horseshoe-shaped, in 

 others nearly circular. The tentacles are arranged on it 



