CHAPTER II 



Multicellular animals Sex Hydromedusse Hydra- 

 Appearance of nerve cells Medusoids Their 

 nervous system " Eye-spots " Purposive move- 

 ments of Hydra according to stimulus Purposive 

 movements of Medusoids dependent on existence 

 of nervous system Purposive movements of uni- 

 cellular organisms accounted for Development of 

 sensory organs Light as stimulus. 



OUR attention thus far has been fixed on uni- 

 cellular beings with the exception of Volvox, 

 which consists of an aggregation of cells fore- 

 shadowing the structural arrangement of all 

 multicellular animals, since, besides the aggre- 

 gation of individual cells to make up its body, 

 the germinal elements of some of these cells 

 separate from the rest of its substance so as 

 to produce male and female cells. The indi- 

 vidual is no longer reproduced by the simple 

 division of a parent cell into two young beings, 

 but depends mainly on the conjugation of male 

 and female cells. 



