ZOOLOGY OF THE INVEKTEBRATA. 



297 



ectoderm, the endoderm, and between them the mesoderm. The 

 ectoderm consists of flattened cells, covering the outside of the 

 sponge, and lining certain pits or depressions which are pushed into 

 the substance of the sponge, and are termed inter-canal spaces. 

 The openings from the exterior into the inter-canal spaces are termed 

 pores. The inter-canal spaces open on their inside by numerous 

 apertures, into the flagellate chambers. These flagellate chambers 

 are the most characteristic feature of Sponges ; the flagella keep 

 up a constant current of water which passes in at the pores, through 



-- 1 



F^g- 75- — Aurelia aurita. 



1. Mouth. 



2. Circumoral perradial processes. 



3. Tentacles on the edge of the 

 umbrella. 



4. One of the branching perradial 

 canals. There are four of these, 

 and four similar interradial canals 



5. 

 8. 



II. 



An adradial canal. 



The circular canal. 



Marginal lappets hiding lentaculo- 



cysts. 



Gastral filaments. 



