SPONGE SPICULES. 



149 



Some of the cells being provided with hair-like lashes 

 (cilia), which they keep waving to and fro, draw the water 

 into the innermost recesses of the sponge through the 

 minute pores, and expel it again through the large holes, 



Fig- 30. SPONGE SPICULES. 



thus keeping up a constant circulation, by means of which 

 all refuse and bad air are carried off; for though they 

 cannot be said to breathe, they do take up oxygen, and 

 give off carbonic acid.* 



* Professor Huxley compares the sponge with a kind of subaqueous city, 

 where the people are arranged about the streets and roads in such a manner 

 that each can easily appropriate his food from the water as it passes along. 



