THE POLYPIER. 



173 



which, as we have seen, the animal lias power to 

 shut itself up. It is the point of adlierence to the 

 foreign body, to which the polypus fixes itself. 



Polypi reproduce themselves in 

 two different ways — by eggs and by 

 buds. The eggs, or larvae, are 

 lodged on the walls of the only 

 cavity which encloses the body of 

 the animal. At certain periods of 

 the year, they leave the body and 

 float in the water, as we have seen 

 is the case with the sponges also, 

 until they meet with a place upon 

 which they can root themselves. 



We have already observed that ^^'^s- ">o.— Branch of 



Coral with Polvpi 



the sponge, during a greater part imirawu. 

 of its existence, lives a vegetative 

 life. So, when a polypus is once fixed upon a solid 

 body, its base extends. If other animals of the same 

 species join it to form a colony, or if it produce buds, 

 the mass gradually increases. Each jx)lypus secretes 

 a matter which, on hardening, becomes horny or 

 stony, and constitutes the polypier, or polypus tree. 

 Tlie nature and form which the polypier gives to the 

 colony serves to characterise the different kinds of 

 animals of this class. 



An insppcti(.n of the polypier constructed by 



