210 ZOOLOGY 



under the microscope eacli stem appears as a series of 

 hydrantlis placed in zigzag fashion, one beyond the other. 



FIG. 196. Obelia ( ?) , a group of campanularian hydroid colonies. Nat. size. 

 Photo, by W. H. C. P. 



Another common kind is Sertularia, 1 which forms rusty 

 brown threads (Fig. 197). All the hydrantlis of one stem 



occur in one plane and 

 oppositely. 



Both tubulariaii and 

 campanularian hydroids 

 may give rise to jelly-fishes. 

 These jelly-fishes are formed 

 as buds on the hydraiith, and 

 after they become able to 

 move of themselves they are 



FIG. 197. Sertularia, a small colony. 



1.5 nat. size. cut oft from the parent and 



1 Derived from serta, garland. 



