FRESH-WATER POLYZOA. 255 



the exterior, arranged alternately on either side, the 

 youngest being at the lower end." 



Each Polyzoan seems capable of producing only a 

 limited number, yet the Polyzoa forming even a single 

 colony are so numerous that the number produced is 

 in the aggregate something astonishing. Their mode 

 of escape by the death of the parent has already been 

 mentioned. 



The winter eggs of each Polyzoan genus are charac- 

 teristic, and from the appearance of even a single one, 

 if mature, its generic origin may be ascertained. It 

 is always a satisfaction to know a thing when it is seen, 

 and to be able to give a positive answer to the oft-re- 

 peated question, "What is it ?" And these little 

 brown specks are sooner or later sure to bring out that 

 question, either from the observer himself or from his 

 friends. By means of the subjoined Key and the fig- 

 ures of the statoblasts in the text, the origin of these 

 winter eggs may be determined. In the figures A 

 points out the annulus. 



Key to the Statoblasts of the Fresh-water Polyzoa. 



A. Reproduction probably by the urn-shaped seg- 

 ments of the stem, Urnatella, Fig. 172. 

 A. Reproduction by 'Statoblasts {a), 

 a. Statoblasts without spines (^). 



a. Statoblasts with spinous margins (d). 



b. "Without a cellular annulus, i^«^<?r«Vi?//«, Fig. 173. 

 b. With a cellular, dark-brown annulus (c). 



b. With a cellular, purplish-blue, iridescent annulus, 



Paludicella. 



c. Extremities rounded, Plumatella, Fig. 174. 



