168 THE MECHANICAL FUNCTIONS. 



without the risk of being broken. This is the 

 case with the Isis hippiuis, commonly known by 

 the name oi jointed coral. (Fig. 68.) There is, 

 in short, hardly any possible combination of these 

 parts which does not occasionally occur amidst 

 the infinite diversities of condition displayed in 

 this department of the animal creation. 



These structures are generally attached to 

 submarine rocks by an expansion of the base 

 into a kind of foot, or root, which has a strong 

 power of adhesion. In this respect, therefore, as 

 in so many others, these animals preserve an 

 analogy with plants. 



It has been ascertained that, in a great number 

 of instances, these fixed zoophytes are mul- 

 tiplied, like the sponge, by the detachment of 

 gemmules, or imperfectly formed portions of 

 their soft substance. These gemmules require 

 to undergo the same kind of metamorphosis in 

 order to bring them to their perfect state ; and 

 when newly detached from the parent, they ex- 

 hibit the same singular spontaneous motions, 

 buoying themselves in the water, and swimming 

 in various directions, by the rapid vibrations of 

 their cilia, till they find a place favourable to 

 their growth. On becoming fixed, they spread 

 out to form a base for the future superstructure ; 

 and, after the foundation has thus been laid, 

 they proceed in their upward growth, depositing 

 a calcareous or horny axis in successive layers, 



