MATEEIALS FOR A MONOGRAPH OF THE ASCONS. 543 



contains in addition considerable quantities of otiier inorganic 

 constituents^ amongst which sodium^ magnesium, and sulphuric 

 acid are demonstrated, together with, probably, water" (1887, 

 p. 132). "The spicules of calcareous sponges are mixed 

 crj'stal individuals, consisting principally of calcite, and con- 

 taining no organic substance ; ^ their outer form, not limited 

 by the surfaces of a true crystal, is conditioned by the specific 

 activity of a living organism, and their inner structure, though 

 completely crystalline, is in relation with the outer form by a 

 peculiar distribution of the constituent parts ^^ (p. 134). "The 

 calcite first secreted contains the greatest amount of impurities 

 (the central thread), and as the tip of the spicule continues to 

 grow a substance corresponding to the central thread is first 

 formed '' (p. 133). 



IV. Theoretical: on the Origin and Evolution of 

 Calcareous Sponge Spicules. 



The spicules of sponges, and especially of calcareous sponges, 

 are structures which seem in a certain sense to connect the 

 two worlds of living and dead matter. Composed, as a rule, 

 of simple inorganic materials, they show the geometrical regu- 

 larity of form which we associate with the mineral kingdom. 

 Formed, on the other hand, within a living body as a product 

 of its vital activity, they exhibit a diversity of character and a 

 progressive evolution of form such as we find only in living 

 organisms. As in the case of crystals, we can reduce them to 

 a few fundamental types, constructed according to simple 

 geometrical patterns. As in the case of living bodies, on the 

 other hand, we can distinguish in them the characters of 

 classes, orders, or families, until we come down to specific 

 characters, and finally to racial and individual variations. 

 Bodies which lie so close in many ways to the border-line 

 between the organic and the inorganic must excite an interest, 

 which is a sufficient justification for embarking upon a dis- 

 cussion as to their nature, and for considering the causes which 

 ' For further remarks on this point see Addendum A (infra, p. 569). 



