is6 



PHYLUM PORIFERA — SPONGES 



CS»^. 



they form buds, the outcome of surplus nourishment. 



These buds, Hke the suckers of a rose-bush, often 



acquire some apparent 

 independence, and the 

 sponge looks like many 

 vases, not like one. 

 Moreover, as they 

 grow these buds may 

 fuse, like the branches 

 of a tree tied closely 

 together. Thus the 

 structure becomes 

 more intricate. 



{b) In the simple 

 sponge the cavity of 

 the vase is completely 

 lined by the collared 

 flagellate cells {Ascon 

 type). But the inner 

 layer may grow out 

 into radial chambers 

 to which the choano- 

 cytes are restricted 

 (Sycon type), and the 

 walls of these may also 

 be folded into side 

 aisles {Leucon type). 

 The outgrowing of the 

 inner layer into the 

 mesogloea may be con- 

 tinued even further, 

 and the cells may 

 become pavement-like 

 except in the minute 

 flagellate chambers, 

 where alone the char- 

 acteristic choanocytes 

 are retained (see Fig. 



76). 



It may be that the characteristic folding or outgrowth 

 of the inner layer is necessitated by the fact that the com- 



FiG. 76. — Diagram showing types of canal 

 system. — After Korschelt and Heider. 

 The flagellate regions are dark through- 

 out, the mesogloea is dotted, the arrows 

 show the direction of the currents. All 

 the figures represent cross-sections 

 through the wall. 



A. Simple Ascon type {Ec, outer layer ; En., inner 



layer ; Mg., mesogloea). 



B. Sycon type, with flagellate radial chambers {r.c). 



C. Leucon type, with flagellate side aisles on the 



main radial chambers. 



D. Still more complex type, with small flagellate 



chambers (f.ch.). 



