100 THE MECHANICAL FUNCTIONS. 



the parent sponge ; assuming circular arrange- 

 ments, and presenting distinct openings at the 

 points they enclose. The young animal now 

 rapidly spreads and enlarges in every direction, 

 becoming more convex, and at the same time 

 more opaque, and more compact in its texture ; 

 and before it has attained the tenth of an inch 

 in diameter, it presents, through the microscope, 

 a miniature representation of its parent. 



Thus has a power of spontaneous, motion been 

 given to what may be regarded as the embryo 

 condition of animals, which are afterwards so 

 remarkable for their inertness, and for the priva- 

 tion of all active powers : and this has been con- 

 ferred evidently for the purpose of their being 

 widely disseminated over the globe. Had not this 

 apparatus of moving cilia been provided to the 

 gemmules of such species as hang vertically 

 from the roofs of caves, they would have sunk 

 to the bottom of the water and been crushed 

 or buried among the moving sand, instead of 

 supporting themselves while carried to a dis- 

 tance by the waves and tides of the ocean. 

 Many species which abound in the Red Sea 

 and Indian Ocean have, in this way, been 

 gradually transported, by the Gulf stream, from 

 the shores of the east to corresponding latitudes 

 of the new world. 



