PROPAGATION OF SPONGES. 115 



canals, forming innumerable minute bud-like points. These, as 

 they increase in size, are gradually clothed with vibratile cilia, 

 and finally detaching themselves are cast out through the 

 oscula into the world of waters. Here their wanderings con- 

 tinue for a short time, until, if they be not devoured on the 

 way, they reach some rock or submarine body, on which, tired 

 of their brief erratic existence, they fix themselves for ever, 

 and bidding adieu to all further rambles, lead henceforth the 

 quiet sedentary life of their parents. 



In this manner the sponges, which otherwise would have been 

 confined to narrow limits, spread like a living carpet over the 

 bottom of the seas, and in spite of their being utterly defence- 

 less maintain their existence from age to age. At the same time 

 they serve to feed a vast number of other marine animals, for 

 the waters frequently swarm with their eggs, and these afford 

 many a welcome repast to myriads of sessile shells, worms, 

 polyps, and other creatures small or abstemious enough to be 

 satisfied with feasting on atoms. 



i 2 



