30 



MULTICELLULAR ORGANISMS 



that serve as food. This nutritive material is con- 

 veyed by a movement of the tentacles into the hollow 

 interior of the body, and is there dissolved by the action 

 of a secretion from the cells of the inner layer that are 

 thus digestive elements. Reproduction is effected by 

 eggs and sperms formed from certain outer layer cells, or 

 by hollow outgrowths of the body wall that, developing 

 a mouth and tentacles, finally pinch off from the parent 

 and become independent organisms. 



Fio. 7. — Freshwater Hydra, ontiro and longitudinally split indi- 

 viduals, f, egg; m, mouth surrounded by tentacles; s, sjwrms, con- 

 siderably magnified. 



Division of Labor. — It is probable that the number 

 of colls in many Vol vox colonies is greater than in Hydra, 

 and yet Hydra is higher in the scale of living things. 

 The reason for this is a difference in the complexity of 



