172 



THE BOTTOM OF THE SEA. 



species, with a tough sheath from which it projects 

 itself at will. Fig. 30 represents a number of such 

 polypi indrawn — that is to say, with the body of the 

 polypus retracted and hidden under the protecting 

 envelope which surrounds its base. 



Fig. 29. — Coral with Polypi more or less expanded. 



The polypi, whose digestive apparatus is formed of 

 a single pouch, are called hydras or sertularias ; those 

 whose digestive apparatus is formed of a double 

 pouch, are corallines, or polypi proper. 



The inferior extremity of the polypus is the pro- 

 longation of that envelope, but more hardened, in 



