174 



PRINCIPLES OF ANIMAL BIOLOGY 



Species which, hke Obelia, exhibit several forms of body are said to be 

 polymo7-phic (hterally of many forms) ._ In ObeHa, as in many other 

 hydroids, polymorphism is accompanied in the life cycle by an alter- 

 nation of asexual and sexual reproduction. The medusae, which are 

 of separate sexes, produce eggs and spermatozoa. The fertilized egg, 

 after fertilization, produces a larva or planula (Fig. 144). This settles 



yC?y7 eumatophore 



Swimmino 

 ~~5e// 



Sensor y 

 ^dividual 



ophore 



I'lti. 145. — Diagram of a siphonophore colony composed of six kinds of individuals. 



fied from Fleischmann.) 



(Modi- 



down, grows into a polyp which buds off other polyps. The colony 

 thus formed also buds off gonangia, whose contained blastostyles bud 

 off medusae. These medusae are set free, and the cycle starts o\-er again. 

 In this cycle the medusae reproduce sexually; all other reproduction in 

 it is budding, that is, asexual. It should be noted that the sexual indi- 

 viduals have a very different structure from the asexual ones. Such a 

 combination of polymorphism with sexual and asexual reproduction is 

 called metagenesis. 



