FRESH-WATER POLYZOA. 445 



being placed in a circle round the mouth, are supported upon 

 a pair of long arms, which usually form a sort of horse-shoe. 

 These are called LOPHOPODA ; their tentacles are usually more 

 numerous than in the marine forms. An example of this struc- 

 ture is seen in the Plumatella (Fig. 705). Xheir polyzoaries are 

 generally membranous, formed of isolated transparent cells, 

 springing from a delicate root-stock, which runs along over the 

 surface of stones and aquatic plants. In the Cristatella, however, 

 the colonies float freely in the water, and consist of numerous 

 individuals enclosed in a gelatinous mass. 



VOL. II. 1 1 



