PROPAGATION OF SPONGES. 115 



in size they are gradually clothed with vibratile hairs 

 (cilia) ; and at length, being fully formed, fall off as oval 

 bodies ; not inert, like the eggs of more active animals, 

 or like their parents, but moving freely 

 by the perpetual vibration maintained 

 by their cilia. These cilia, by their united 

 action, create strong currents round the 

 little body, which drive it forward into K00 OF 8PONOJ . 

 the stream that issues from the opening 

 of the Sponge, and thence into the open sea, where its 

 motion is continued till it has reached a place suitable 

 for its development. When this is done it soon attaches 

 itself; its wanderings cease, and it commences the quiet 

 vegetative life of its parent. The instincts which guide 

 animals in the care of their young are among the most 

 interesting that the lower animals exhibit ; but here, at 

 the base of the scale, we find a passive parent whose 

 young are endowed with powers of motion denied to 

 its mature growth, and these obviously supply, by a 

 beautiful arrangement, the deficiencies of the mother. 

 When we look a little higher in the animal scale, we 

 shall find other instances of greater activity in the young 

 than in the mature animal ; and even among the lower 

 vegetable tribes, the spores are often endowed with pro- 

 per movements. The little seed-like bodies from which 

 the Alga? spring, are, in many instances, clothed with 

 cilia, like the eggs of the Sponges, and enjoy, for a brief 

 period, a similarly active life. The animal egg of the 

 Sponge and the vegetable egg of the Conferva are both 

 moved by the same agency, and each appears to select 

 the situation best adapted for its growth. The phases 



