306 



ESSENTIALS OF ZOOLOGY 



its distal end is the mouth, bordered by four oi^al lobes. The mouth 

 is the aperture leading into the internal or gastrovascular cavity 

 which has four radial branches or canals. 



Obelia is a marine, colonial type resembling a branched plant in 

 appearance. The individuals are attached to each other in the 

 colony, and it is fastened to a rock or other substratum by a root- 

 like hydroi^hiza. They are distributed in the Atlantic Ocean and 

 Gulf of Mexico out to forty fathoms in depth. The colony begins 



Medusa 



Mature 

 (jonancjium. 



Position of. 

 mai 



sjtion Of 

 rture colony 



//ew 

 colony 



Sperm from 

 another 

 medusa 



(o^^_ .Terbilijed ecja 

 V i^ycjote) 



\^ ^ ^^> Cleovacfe 



Blastula.^ 



'^ \arva ^ 



Fi,?. 106. — Life cycle of Ohelia, illustrating polymorphism and metagenesis. Adult 

 hydroid colony with mature gonangium gives rise to sexual medusa which is pro- 

 duced in the gonangium and set free in the water. Germ cells produced by the 

 medusae complete the cycle. Blastula and planula stashes are free swimming. (Re- 

 drawn and modified from Wolcott, Animal Biology, McGraw-Hill Book Company, 

 Inc.) 



as a single individual which buds, but they do not separate from the 

 preceding or parent generation. This may continue for several gen- 

 erations. 



The reproductive cycle is both sexual and asexual, alternating 

 between the sexually produced polyp or hydroid generation and 

 the asexually produced sexual generation, the medusa or jellyfish 

 form. The medusae arise as buds from the special individuals, 



