152 THE MECHANICAL FUNCTIONS. 



infinite diversities of condition displayed in this 

 department of the animal creation. 



These structures are generally attached to sub- 

 marine 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 multiplied, 

 like the sponge, by the detachment of gemmules, 

 or imperfectly formed portions of their soft sub- 

 stance. 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 exhibit 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 super- 

 structure ; and, after the foundation has thus been 

 laid, they proceed in their upward growth, deposit- 

 ing a calcareous or horny axis in successive layers, 

 until it has acquired the requisite thickness ; * and 

 they then gradually assume the forms characteristic 

 of the particular species to which they belong. The 



* The materials thus deposited have been usually considered as 

 being permanent structures, not capable of removal or modification, 

 and not possessing vital properties ; such properties being supposed 

 to belong exclusively to the animated flesh with which these struc- 

 tures are associated ; but the recent inquiries of Milne Edwards afford 

 reason for the belief that, in some cases at least, the horny and even 

 calcareous parts of zoophytes are not wholly inorganic, that they 



