168 GENERAL ZOOLOGY 



Race Preservation. Obelia has a rather complicated life 

 cycle because there is always an alternation of polyp and 

 medusa. The polyp generation reproduces by asexual 

 methods. It starts as one individual which produces a great 

 number of others by budding ; some remain attached to the 

 colony and others are set free. The medusa produces 

 zygotes which grow into young and vigorous polyps. Such 

 an alternation of a generation which reproduces asexually 

 by budding with another generation which produces new 

 individuals by sexual methods is known as metagenesis. 



Metagenesis in Obelia takes the following sequence (Fig. 

 71): The zygote (A) develops in the way described for a 

 typical metazoan, passing through blastula and gastrula 

 stages, and finally forms a ciliated larva (B) which swims 

 about for a time, then attaches itself to some object along 

 the shore. The larva is transformed into a little polyp 

 (C), which soon starts to feed and forms a peritheca about 

 itself. As it grows the polyp buds off hydranths and 

 gonangia until a great branching colony is formed (D). 



When the gonangia on a colony are mature, they are 

 continually forming medusa buds (Fig. 67, &). These 

 ripen into little medusse (Fig. 71, E) which break loose and 

 swim out through the opening at the end of the gonangium. 

 A medusa swims by pulsating its umbrella, and is able to 

 feed on small animals. It swims feebly, however, and is 

 often swept a long way by ocean currents. Though some 

 individuals may die before the ends of their journeys, such 

 traveling is of benefit in general, for Obelia is thus able to 

 become established in new localities. Each medusa is 

 either a male or female. The females give off zygotes 

 formed by the union of egg and sperm cells and the cycle 

 thus begins again. 



In metagenesis, then, as illustrated by Obelia, there is 

 always an alternation of an asexual polyp generation with 

 a sexual medusa generation. Polyps cannot form other 

 polyps directly except by budding and medusae must 

 always produce germ cells which grow into polyps. At first 



