THE SLIGHTLY COMPLEX ANIMALS OR SPONGES 39 



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. 17). 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. 17 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. 8, 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 . 



r . J FIG. 16. One of the Sim- 



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 

 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 



plest sponges ( Calcolyn- 

 thus primigenius (after 

 HAECKEL). A portion 

 of the wall has been re- 

 moved to show the in- 

 side. 



