FRESH-WATER POLYZOA. 241 



tic or horseshoe-shaped disk of tentacles. In the 

 largest, and therefore the oldest, colonies the jelly 

 may exhibit many scattered white spots composed of 

 carbonate of lime. 



There is another jelly-forming colony called Crista- 

 te'lla, 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. Onespe'cies 

 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, yet moves 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 as on a base, usually in a double row. The word 

 is Greek, and means "wearing a crest." 



In those Polyzoa which secrete hardened, tubular, 

 tree-like sheaths on the surface of submerged objects, 

 the lophophore is protruded from the orifice in the end 

 of the branch much as in Pectinatella, and there is 

 only one animal to each limb or hollow twig. The 

 protrusion and expansion of the lophophore can be 

 seen with a pocket-lens, as in Fig. 169 (from Professor 

 Alpbeus Hyatt's work on the Polyzoa), when it resem- 

 bles i-n form that of Pectinatella. Those inhabiting 



17 



