416 



RADIATA. 



The mode by which the Sponges increase their 

 race is highly curious : at stated periods, there pro- 

 ject from the interior of the large canals yellowish 

 buds, which grow until at length they are detached, 

 and are driven with the perpetual torrent into the 

 surrounding sea. Here one would naturally expect 

 that so apparently helpless an atom of jelly would 

 sink to the bottom at once by the side of its parent. 

 But if this were the case, it is easy to perceive that 

 one particular spot would be loaded and overgrown 

 with Sponges, which would clog and choke each other, 

 while the waste around would be destitute of any. 

 To remedy this, a power of motion is given to the 

 tiny gemmule, of which the parent is totally deprived. 

 The microscope discovers the greater part of its 

 surface to be covered with minute cilia, which by 

 their rapid vibrations propel it swiftly along through 

 the sea, until being arrived at a sufficient distance 

 from the place of its birth, it quietly settles on the 

 bottom, loses its cilia, and acquires the form and 

 character of a Sponge. 



Many Sponges are remarkable for the singularity 

 of their forms, sometimes spreading into broad leaves 

 or fans ; sometimes cylindrical like a thick stick ; 

 now assuming the form of a hand; and often hol- 

 lowed out in the shape of a cup, with a foot 

 like a vase. They occur abundantly on all shores 

 from Greenland to Australia, but are most numerous 

 and gigantic in hot latitudes. The Common Sponge 

 of commerce, is chiefly found in the Mediterranean, 

 where, especially among the beautiful Isles of 



