REFERENCES 
159 
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 endoderm, 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 protozoon do? Compare with the Colo¬ 
nial 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. 
