io8 



PORIFERA. 



In the phylum there can be discerned at least four 

 different types of sponges according to the distribution of 

 the choanocytes. 



(i) Ascon type. In this simplest type the whole para- 

 gastric cavity is lined by choanocytes and there are no 

 radial canals (Fig. 39). 



(2) Sycon type. The choanocytes are restricted to the 

 exhalent radial canals ; inhalent canals and prosopyles are 

 present (Fig. 42). 



(3) Leucon type. The choanocytes are restricted to a 

 number of secondary radial canals opening into the primary 

 radial canals (Fig. 43). 



Fig. 43-- 



-Transverse Section of Part of the Wall 

 OF A Leu CON. (Diagrammatic). 





Paragasiiic 

 Cavity. 



-■ Exhalent Canal. 



-Pore. 



'Secondary In- 

 halent Canal. 



Secondary Exhal- 

 ent Canal, with 

 Choanocytes. 



(4) Rhagon type. The secondary canals are contracted 

 into small round chambers and only open into the primary 

 radial canal by a number of exhalent canals or apopyles 

 (Fig. 44)- 



