16 



ZOOLOGY. 



Fig. 13.— a, longitudinal section through a simple calcareous sponge, showing its 

 simple central cavity; 6, showing a single osculum at the top, and the many 

 mouths over the surface. After Haeckel. 



Fig. 14.— Microscopic section of a bit of sponge. 1, canals leading to the cham- 

 bers; 2, section of the same enlarged; 3, the ciliated cells, highly magnified. 

 From Liitken's Zoology. 



