STRUCTURE OF SPONGES. 127 
easily and well. Like plants, they form buds, the outcome 
of surplus nourishment, 
a rose-bush, often ac- 
quire 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 to- 
gether. Thus the struc- 
ture becomes more in- 
tricate. 
(6) 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 
minute flagellate cham- 
bers, where alone the 
characteristic choano- 
cytes are retained (see 
Fig. 60). 
It may be that the 
characteristic folding or 
‘These buds, like the suckers of 
Fic. 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. 
Simple Ascon type (Zc., outer layer; Az., 
inner layer; /Zg., mesoglcea). 
Se type, with flagellate radial chambers 
ree 
the main radial chambers. 
Still more complex type, with small flagel- 
late chambers (/.ch.). 
A. 
B. 
C. Leucon ‘type, with flagellate side, aisles on 
D. 
outgrowth of the inner layer is 
necessitated by the fact that the component cells are better 
