CHAPTEE III. 

 THE SPONGES. 



It 18 a great step from an inJusorian to a sponge — ^from a 

 one-celled to a many-celled animal. Formerly sponges were 

 supposed to be plants, but they are now known to be com- 

 posed of numerous cells, arranged in three layers, and to 

 develop from eggs like the higher animals. 



A sponge, then, is a cellular sac (Fig. 16) with digestive 



FlQ. 16. — A longitudinal section through a simple calcareous sponge, showing the 

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

 mouths over the surface. 



chambers or minute rude stomachs lined with ciliated cells, 

 the whole sponge-mass being supported by an irregular 

 basket-work of needle-like bodies called spicules (Fig. 17). 



