32 The Under-Water World 



with innumerable wavy hairs which 

 violently beat the water, thus propelling 

 the infant sponges from point to point. 

 After a varying period the " spongeling " 

 settles down on a coral reef, on a dock 

 pile, or even on a living crab, and from 

 thence onwards grows by expansion- a 

 slow but steady multiplication of its 

 cells. Many forms build around them- 

 selves ramparts composed of millions of 

 minute spicules of carbonate of lime, 

 silica, or horny substance which fre- 

 quently mimic with wonderful fidelity 

 such human inventions as the barred 

 arrow head and the ship's anchor. The 

 surface of the adult sponge is perforated 

 by innumerable pores through which 

 water enters, conveying the necessary 

 food and oxygen for the purpose of 

 respiration. A large hole at one end of 

 the vase-shaped body serves as an exit 

 for the water current which has been 

 taken in through the myriads of minute 



