270 UTILIZATION OF MINUTE LIFE. 



The ova of sponges are numerous irregularly- 

 shaped granular bodies, endowed with vibrating 

 cilia, by which they move. They issue at different 

 periods from the gelatinous matter. These ova 

 float in the water ; moved about by the cilia which 

 garnish their anterior extremity, they are carried on 

 by the currents through the sponge, and are finally 

 expelled through the larger orifices. They swim 

 about freely in the water for a little while, and then 

 fix themselves for ever to the rocks, and grow into 

 new sponges. These ova, or moveable eggs, have 

 frequently been taken for the animal (the sponge) 

 itself. 



The spicula are microscopic needles, sometimes 

 straight, sometimes curved or star-shaped; others 

 resemble the anchors of ships, etc., in form. When 

 the spicula are siliceous, they are best seen after 

 the sponge is burnt, on examining under the micro- 

 scope the ash which is left. 



Sponges with calcareous spicula are rather nu- 

 merous on our coasts, and siliceous spicula are 

 common in sponges of most latitudes. 



It is almost entirely to English naturalists that 

 we are indebted for the knowledge we possess of 

 these curious organisms. Ellis was the first to 

 establish the existence of currents of water passing 

 constantly through the tissue of sponges. Dr. Grant, 

 whilst confirming Ellis' s observation, added so much 



