68 THE STORY OF ANIMAL LIFE 



substance. The cells of the sponge, however, 

 wear away the lime of the shell by means of 

 some acid chemical action. Not only so, but 

 they can attack stones as well, when these con- 

 sist of limestone ; and on some parts of the 

 coast bits of sponge-eaten limestone washed up 

 on the beach are quite common objects. They 

 are pierced all through by holes, so that their 

 appearance would suggest a sponge carved in 

 stone, but for the fact that the holes are fairly 

 uniform in size. Such stones, lying on the shore, 

 often puzzle the finder, when they contain no 

 apparent trace of the tenant that has worked its 

 way through them. 



The sponges have received the name of 

 Porifera, on account of the structure above 

 described. They are often classed with the 

 Ccelenterata, because, among other reasons, they 

 practically belong to the two-layered type of 

 structure, and because they form a complex 

 organism that may almost be called a colony. 

 But some prefer to place them in a group by 

 themselves, apart from the Ccelenterata. The 

 chief reason of this is that the sponges, as com- 

 pared with a primitive two-layered type indicated 

 by their own larvae, are turned upside down, the 

 mouth being, as above stated, originally situated 

 at the fixed end. 



CHAPTER VII 



WORMS 



WHEN the great naturalist, Linnaeus, framed 

 his classification of the animal kingdom, he in- 



