124 



/'// \L UM FOR I PER A S PONCES. 



A 



as they grow these buds may fuse, like the branches of a 

 tree tied closely together. Thus the structure becomes 



more intricate. 



(I)) 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 

 (Sycon type), and the walls 

 of these may also be folded 

 into side aisles (Leucon 

 type). The outgrowing 

 of the inner layer into the 

 mesoglcea may be con- 

 tinued even further, and 

 the cells may become 

 pavement-like, except in 

 the minute flagellate 

 chambers, where alone the 

 characteristic choanocytes 

 are retained (See Fig. 57.) 

 [Speculatively, it may 

 be suggested that the 

 characteristic folding or 

 outgrowth of the inner 

 layer is necessitated by 

 the fact that the com- 

 ponent cells are better 

 nourished and multiply 

 more rapidly than those 

 of the outer layer.] 



(c) By infoldings of the 

 outer layer and a sub- 

 jacent sheath of meso- 

 glcea - - subdermal spaces 

 may be formed ; an outer 

 cortex may be distinctly 

 differentiated from the internal region in which the flagellate 

 chambers occur ; the pores may collect into sieve - like 



V 



FIG. 57. Diagram showing types 

 canal system. After Korschelt 

 Heider. The flagellate regions 



mesoglcea 



of 

 and 

 are 



is 



dark throughout, the 

 dotted, the arrows show the direction 

 of the currents. All the figures re- 

 present cross-sections through the wall. 



A. Simple Ascon type (c., outer layer ; n., 



inner layer ; Mg. , mesoglcea). 



B. Sycon type, with flagellate radial cham- 



C. Ztype with flagellate side aisles 



on the main radial chambers. 



D. Still more complex type, with small 



flagellate chambers (f.ch,). 



