THE EUPLECTELLA 67 



the most favourable conditions for resorption by 

 organisms living upon the layer of protoplasmic 

 substance which covers the bottom of the ocean, 

 into which the silica has been received. As Wallich 

 and others have most distinctly proved, this proto- 

 plasmic layer, where it exists, is the product of the 

 organic life, and not its source. 



This gives us an insight into the conditions pre- 

 vailing on portions of ocean floors not affected by 

 tides and currents, and accounts for the large quanti- 

 ties of glass sponges and their supplies of silica. 



In fact, it throws light upon many other interesting 

 problems connected with ocean life, and in no small 

 measure aids us in the study of rocks, such as those 

 of the Jurassic formation. This must be our excuse 

 for quoting at such length. 



This chapter contains seven illustrations out of 

 many that could have been supplied. The seventh 

 shows, in enormously enlarged forms, the spicules of 

 1 various sponges.' 



The sponges themselves vary from nine to fourteen 

 inches in length. 



The last point, and a very wonderful one it is, 

 remains to be noticed. Long ago geologists were 

 acquainted with beautiful objects which occupy the 

 centres of certain flints found on the sea-shore and 

 other places, and, not knowing their history, they 

 called them Ventriculites. They are found in a 

 variety of shapes, such as balls with stems, cornu- 

 copia forms, others are like wine glasses (see fig. 58), 



