THE SPONGES 65 



ganisms that it contains; on these the sponge 

 feeds. 



It will naturally be asked, how does this liv- 

 ing filter work ? Water will not pass through 

 small holes to flow out again at large ones in an 

 upward direction, unless helped by some mechan- 

 ism. How is this supplied ? By the industry of 

 the cells of the sponge. Its canal-system includes 

 a set of wide chambers, lined with cells which 

 have long cilia, called flagella. These flagella, 

 constantly moving in one direction (like the fan 

 of a ventilator), create a current, which passes 

 the water on with such force that it reaches the 

 central cavity, whence it is expelled through the 

 oscula. These chambers do not communicate di- 

 rectly with the exterior. They are closed, except 

 at certain small holes, the " prosopyles," where 

 they take in the water that enters from spaces 

 connected with the pores. At the main end of 

 the chamber is an aperture called the " apopyle," 

 capable of being partly closed, and leading into 

 an excurrent passage. This last communicates 

 with the central cavity of the sponge. 



It "will be seen that the topography of the 

 sponge is a very complicated business. All its 

 details have been studied by means of thin sec- 

 tions specially prepared and placed under the 

 microscope (see p. 183) ; in these the labyrinth 

 of canals and chambers is seen cut through at 

 various points ; the cells lining them and divid- 

 ing them may be individually studied. The pas- 

 sage of water through the sponge was first ob- 

 served by Robert Grant ; many of the most recent 

 discoveries regarding the structure of sponges we 

 owe to Professor Sollas. 



We have not yet explained what our living 



