SPONGES. 145 



find an analogy between the constitution of zoo- 

 phytes and that of plants. 



In the course of a few weeks, the spicula are 

 assembled in groups, similar to those of the parent 

 sponge ; assuming circular arrangements, and pre- 

 senting .distinct openings at the points they enclose. 

 The young animal now rapidly spreads and en- 

 larges in every direction, becoming more convex, 

 and at the same time more opaque, and more com- 

 pact 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 re- 

 markable for their inertness, and for the privation 

 of all active powers ; and it has been conferred 

 evidently for the purpose of their being widely 

 disseminated over the globe. Had not this appa- 

 ratus 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 distance 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 old to corresponding latitudes of the new 

 world. 



VOL. r. 



