108 PORIFERA. 
In the phylum there can be discerned at least four 
different types of sponges according to the distribution of 
the choanocytes. 
(1) 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 Leucon. (Diagrammatic). 
Paragastric 
Cavity. 
i 
einai, In- 
R 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 afopyles 
(Fig. 44). 
