Gonad 



Hydranth 



Medusa 



from male 



colony 



BLASTULA 7 



OOI 



j/ PLANULA 



MATURE FEMALE 

 COLONY 



YOUNG COLONY 



Figure 57. Life history and structure of Obelia, a colonial marine hydroid. The colony consists 

 of polyps of two types, the feeding hydranths and reproductive gonangia. Both hydranths and 

 gonangia are developed by asexual budding from stems attached to a branching rootlike tangle 

 called the hydrorhiza. Sperms and eggs are produced by medusae which bud from gonangia of 

 different colonies, that is, the colonies are dioecious. The embryo develops into a ciliated free- 

 swimming planula larva; this attaches to the substratum and forms a new colony. The three 

 kinds of individuals — the feeding polyps (hydranths), the asexual reproductive polyps, and the 

 sexual reproductive medusae — illustrate polymorphism. 



11^ 



