268 



ZOOLOGT 



common kind is Sertularia/ which forms rusty brown threads 

 (Fig. 258) . All the hydranths of one stem occur in one plane 

 and oppositely. 



Fig. 



257. — Obelia, a group of campanularian hvdroid colonics. Nat. size. 

 Photo, by W. H. C. P. 



Both tuliularian and campanularian hydroids may gi^'c rise 

 to jellyfishes. These jellj-fishes are formed as buds on the 



hydranth, and after they be- 

 come able to move of them- 

 selves they are cut off from the 

 parent and swim away." The 

 jellyfishes then give rise to the 

 S(>xual products and discharge 

 them into the water (Fig. 260). 

 As in this case the life-history 

 of tlie animal consists of an 

 attached individual alternating 



Fig. 258. — Scrtularia, a small colony, 

 1.5 nat. size. 



' Derived from serta, garland. 



= Fig. 259. 



