62 THE SIMPLER METAZOA 



SUMMARY 



The transition from simple Protozoa, through the Colo- 

 nial Protozoa, to the Metazoa is simple and direct. In 

 gonium and volvox, the beginning of division of labor is 

 noticed ; that is, one part of the body becomes dependent 

 on another part for certain definite things. For example, 

 one cell is devoted to securing food, while another produces 

 eggs or sperms. The sponges are simple Metazoa in which 

 the division of labor has taken the form of producing 

 three layers, — the ectoderm, or outer layer ; the endo- 

 derm, or inner layer; and a loosely formed middle layer. 

 Grantia is a simple sac-shaped sponge which reproduces 

 both sexually and asexually. The general manner of 

 development by the sexual process is essentially the same 

 in all the higher animals, including man. The bath 

 sponges are the only ones of economic importance. 



QUESTIONS 



What can the single-celled protozoan do ? Compare with the Colonial 

 Protozoa, gonium and volvox. Explain the meaning of division of labor 

 in an animal. In what respects do sponges differ ? Of what use are 

 they ? Why are not all sponges useful ? 



REFERENCES 



Hegner, Introduction to Zoology, Chapter VI. 

 Jordan and Kellogg, Animal Life, Chapter II. 

 Osborne, Economic Zoology, Chapter III. 



