398 ZOOLOGY. 



13. Polymorphism. Closely associated with this ques- 

 tion is the phenomenon of polymorphism, which may be 

 denned as the existence of individuals of different form 

 and structure in the same species. This is distinctly 

 exemplified in Obelia. The medusoid and the hydroid are 

 both produced from the same parents, but they have a 

 different structure and different organs. In the gem- 

 marium, again, the blastostyle is different from the 

 feeding zooids. In allied forms such different zooids are 

 distinguished as trophozooids and gonozooids. Obelia, 

 therefore, may be said to be trimorphic. In the gem- 

 marium polymorphism is connected with the specialisation 

 of the zooids and the development of the individuality of 

 the gemmarium ; from the latter point of view the ordinary 

 zooids are feeding organs, the blastostyles the reproductive 

 organs. Thus the relation between the medusa as an 

 independent zooid and the gemmarium as a compound 

 consisting of many zooids is somewhat similar to that 

 between a Protozoan consisting of one cell and a Metazoan 

 which is a compound of many cells. In Herbert Spencer's 

 language, if we take the cell as the unit, then an ordinary 

 individual such as the Medusa is an aggregate of the first 

 order, and the gemmarium an aggregate of the second 

 order. 



