PORIFERA 233 



entoderm, and a middle gelatinous connective- tissue matrix 

 in which are embedded branched connective-tissue cells, 

 calcareous or silicious spicules, primitive muscle cells and repro- 

 ductive cells. Through the walls of the sponge numerous fine 

 pores or slender canals penetrate from the exterior to the 

 central larger canal or cloaca. In the simpler sponges the 



FIG. 132. A Niaxon sponge (Pheronema?). Philippine Islands. X 1/2. 



surface of the cloaca is lined with the collared flagellate cells. 

 These have a marked resemblance to the protozoan Choano- 

 flagellata and are not found in any other metazoa. The 

 lashing of the flagella creates a current in the water which flows 

 inward at the pores and outward at the osculum, the large open- 

 ing of the cloaca. The flagella and collars together serve for 



