ls<; ZOOLOGY. 



inferior extremity of the polyp is contracted, so as to adhere 

 to foreign bodies, on which the animal is destined to live 

 fixed to them ; its skin generally hardens to a large extent, 

 so as to form a horny or calcareous envelope, analogous to 

 the cellules of which we have already spoken in describing 

 the bryozoaria. The polyps, properly so called, resemble also 

 the molluscoids by their mode of multiplication ; for most of 

 them not only reproduce by means of eggs, but also by means 

 of granulations, which spring from different parts of the sur- 

 face of their bodies and never become detached; so that 

 different generations remain engrafted as it were on each 

 other, and form larger or smaller masses, in which all the 

 individuals of the same race are included, and live, up to a 

 certain point, a common life. 



Fig. 470. Polyp of the genus Astroidea. 



The portion, in some measure ossified, of the tegumentary 

 tunic of these polyps, presents varied forms, and constitutes 

 sometimes tubes, sometimes cellules. For a long period this 

 was considered merely as the dwelling of the polyps which 

 form it, and it is to it that the name of polypier has been 

 given. Sometimes each polyp has a distinct polypier, but 

 in general it is the common portion of a mass of aggregated 

 polvpi which presents the characters peculiar to these bodies, 

 and thus these form aggregated polypiers, the volume of 

 which may become very considerable, although each of its 

 constituent parts has dimensions which are very small, 



