232 



CLASSIFICATION OF ANIMALS 



of minute algae imbedded in the tissue of the sponge in a sym- 

 biotic relationship. In size sponges vary from a fraction of an 

 inch to several feet in diameter. 



Fig. 131. — Stylotella heliophila, a typical sponge. Beaufort Harbor, N. C. 



X 1/2. 



520. A sponge is essentially a tubular structure, the walls of 

 which consist of three layers, an outer thin epithelium, the 

 ectoderm, an inner epithelium of collared flagellate cells, the 



