SOLUTION OF SPICULES 



147 



PI 



Sg S 



5*2 



are cylindro-conical, sharp at one end and triradiate at the 



other ; others are needle-shaped, sharp at both ends ; but 



these are the characteristic forms of some sponge spicules, 



and in particular of some of those spicules which we still 



find in the solid state accompanying the flints (Fig. 41) ; 



not only do the hollow cavities precisely agree with these 



bodies in form, but in size also. There is nothing in the 



organic or inorganic w T orld which presents us with these 



forms except sponge spicules, and consequently we cannot 



resist the conclusion that the empty 



spaces having these forms which now 



exist in the chalk were once occupied 



by sponge spicules ; the substance of 



these bodies has disappeared, their 



form alone remains. The silica which 



has been removed is far from being a 



negligible quantity ; it is possible to 



estimate it by measuring under the 



microscope the area of a given surface 



of chalk and of the spicular cavities 



visible upon it. In the case of the 



chalk near Pangbourne railway station 



the hollow casts are present sometimes 



to the extent of 3 per cent. This is a 



sufficiently striking result, but there 



can be little doubt that in the space 



now occupied by the flint nodules themselves, spicules 



were once present in far greater abundance than this. 



Besides these spicules, the form of which is preserved, 

 countless others must have existed which have left no 

 vestige of a trace behind. In existing sponges the 

 elements of the skeleton are of two kinds, which differ 

 markedly in size, and are thus distinguished under the 

 names of megascleres and microscleres ; but it is the 

 larger form, or megascleres, alone which have left their 



lt 



