STRUCTURE OF SPONGES. 



127 



easily and well. Like plants, they form buds, the outcome 

 of surplus nourishment. These buds, like the suckers of 

 often ac- 

 apparent 



a rose-bush, 

 quire some 

 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 to- 

 gether. Thus the struc- 

 ture becomes more in- 

 tricate. 



(F) 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 choan- 

 ocytes are restricted 

 (Sycon type), and the 

 walls of these may also 

 be folded into side aisles 

 (Leucon type). The out- 

 growing of the inner 

 layer into the mesogloea 

 may be continued even 

 further, and the cells 

 may become pavement- 

 like except in the 



FIG. 60. Diagram showing types of 

 canal system. After Korschelt and 

 Heider. The flagellate regions are 

 dark throughout, the mesogloea is 

 dotted, the arrows show the direction 

 of the currents. All the figures re- 

 present cross-sections through the wall. 



minute flagellate Cham- A . Simple Ascon type (Ec., outer layer; En., 

 Where alone the _ 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. 



characteristic choano- 

 cytes are retained (see 

 Fig. 60). 



It may be that the 

 characteristic folding or outgrowth of the inner layer is 

 necessitated by the fact that the component cells are better 



D. Still more complex type, with small flagel- 

 late chambers (/.ch.). 



