18 MECHANICAL FUNCTIONS, 



tinued to watch the same orifice, at short intervals, for 

 five hours, sometimes observing it for a quarter of an 

 hour at a time, but still the stream rolled on with a con- 

 stant and equal velocity." 



The water thus poured forth in a perpetual stream 

 from these apertures is received through the millions of 

 pores which pervade the sponge in every direction, and 

 by this means it is that the animal is nourished. Even 

 fish of several pounds weight will live for weeks, or per- 

 haps months, upon no other nourishment than what is 

 contained in sea water, so that the sustenance of the 

 Sponge by such means presents nothing uncommon. 



The mechanism by which these currents of water are 

 constantly produced, is involved in obscurity. It is 

 however supposed to consist of cilia, or small hairs lining 

 the inner surfaces of the tubes, the motions of which 

 propel the water through them. 



These currents are readily made apparent by placing 

 the living animal in a shallow vessel of sea water, and 

 strewing a little powdered chalk on the surface, the 

 motions of which make that of the water plainly visible, 

 as shown in the figure. 



Manner in which the young Sponges are disseminated. 

 In all parts of creation, whether of the vegetable or ani- 

 mal kingdoms, there is provided effectual means for the 

 dissemination of the species. (For an account of the 

 dissemination of the seeds of plants, see the author's In- 

 troduction to Botany.) For the distribution of the Spon- 

 ges the method provided is singularly curious and interest- 

 ing, and at the same time displays in a most striking 

 manner the care which the Creator has taken to per- 

 petuate his most humble works. 



On examining certain parts of the Sponge, which 

 when living and wet, is nearly transparent, there is 

 found a multitude of yellow opaque spots, visible to the 

 naked eye. These, when examined with a microscope 



Whence comes the water which is poured forth from these apertures ? 

 How does the sponge obtain its nourishment 7 By what means is it 

 supposed the currents through the apertures are produced 1 How are 

 the currents made apparent 1 In what manner are the young sponges 

 distributed 1 



