334 COMMON OBJECTS OF THE SHORE. 



On strewing some powdered chalk on the surface 

 of the water, the currents were visible to a great 

 distance ; and on placing some pieces of cork or of 

 dry paper over the apertures, I could perceive 

 them moving by the force of the currents, at the 

 distance of ten feet from the table on which the 

 specimens rested." And thus the wondrous ope- 

 rations of nature, till lately unsuspected, have been 

 continually going on since man first awoke to light 

 and life ; the knowledge of which reminds us how 

 much there may be still in the world around us of 

 which as yet we know nothing. Surely the philo- 

 sopher may well be the humblest of men, since he 

 best knows the degree of man's ignorance and the 

 finite nature of his understanding. 



The young eggs or gemmules of the sponge are 

 attached to the inner sides of the canals, and are, 

 when fully formed, clothed with the minute hairs 

 which serve so important a part in the economy of 

 many small marine animals. Here again they 

 are useful in forming currents in the water around 

 them, by means of their rapid and incessant mo- 

 tion, and these currents carry away the young egg 

 of the sponge into the sea, till it fixes itself in some 

 rocky crevice, or on some sea-weed or other object, 

 and springs up like a little tree, or forms itself into 

 a ball or cup, or takes any other form, according 

 to its species and the circumstances under which 

 it grows. 



Additional strength is given to the framework 

 of some kinds of sponges by calcareous or flinty 

 minute crystalline spicula, which, small as they 

 are, are capable of scratching glass. The existence 

 of these in our British sponges, as well as the 

 smaller growth of these productions, render them 



