SoLLAs — On Sponge Spicules. 



381 



spicules ; and without they are surrounded by a thin sheath of a 

 probably similar material. I find by calculation that, allowing for 

 the organic matter a specific gravity of 1-5, it would require to be 



Fig. 2. 



Adularia, 



An occasional spicule, . . . 



Middle of zone of calcareous spic, _ 

 Beginning of same zone, .... 



Quartz and perforate foraminifera, 



Perforate foraminifera (lowest lyin 

 examples), 



Calcite and Milliola, .... 



Lowest lying Milliola, 



2.67 2.65 2.63 2.62 



a. Axis of distances. b. Axis of specific gravities. 



present to the extent of 6| per cent, to reduce the total specific 

 gravity of the spicules from 2*7 (supposing them to consist chiefly 

 of calcite) to 2*62, the density found. 



02:>tie Axis and Angles of Extinction. — Lying on the borders of 

 the organic and the mineral worlds, the spicules of calcisponges 

 offer many interesting problems for consideration. Their outer 

 form, moulded by the organism, offers no crystalline outlines; 

 their molecular structure, as will appear from the next series of 

 investigations, is purely crystalline, not differing from that of 

 calcite. 



Haeckel, in his work on the calcisponges, regards the regular 

 triradiate spicules as having a crystalline form, derived from a 

 regular 12-sided pyramid ; but as one cannot derive the angles of 

 the sagittal and irregular triradiates by any known laws of crystal- 

 lography from the more regular forms, one fails to see the grounds 

 for such a view. The existence of precisely similar regular and 

 sagittal triradiates amongst the siliceous sponges in which the 

 mineralizing matter is purely colloidal also makes against this 

 opinion. Were the regular triradiates of Hseckel placed within a 

 scalenohedron in the position his view requires, the optic axis would 

 pass through the centre of the spicule at right angles to its plane, 

 and consequently, on viewing it through crossed nicols, it would 

 remain obscure during a complete rotation about its axis in the 



