THE SPONGES 
25 
floating organisms or organic remains which serve as food. 
The mechanism by which this process is effected, and the 
various other structures of the body, are in large part invis- 
ible from the exterior, requiring the 
study of thin sections of the sponge 
to make them clearly understood. 
Under the microscope such a sec- 
tion shows the body of a sponge to 
consist of an immense number of va- 
riously formed cells constituting three 
distinct layers (Fig. 14). Not only 
do these layers consist of different 
kinds of cells, but the duties per- 
formed by each are different. For ex- 
ample, a glance at Fig. 14 will show 
that in the inner layer certain colum- 
nar cells exist, provided with a fla- 
gellum and encircling collar, the ap- 
pearance being strikingly like certain 
of the Protozoa (Fig. 6, B). During 
life their whip-like processes, lashing 
backward and forward in perfect uni- 
son, produce currents of water which 
continually pass through the body. 
The food thus entering the animal is 
taken up by the cells of the inner 
layer as it passes by. The supply, 
however, is usually more than suffi- 
cient to meet the demands of this 
layer, and the excess is passed on to 
the middle and outer layers. The 
Fig. 13.—One of the sim- 
plest sponges (Calcolyn- 
thus primigenius (after 
HAEcCKEL). A portion 
of the wall has been re- 
moved to show the in- 
side. 
exact method by which this occurs is still a matter of 
doubt, but there seems to be little question but that 
each cell of the body receives its food in a practically un- 
modified condition, requiring that it digest as well as 
assimilate. The oxygen necessary to this latter process 
