66 Mr. a. P. Bidder. 



The four tli or gastral ray of a quadriradiate spicule is an acute 

 rhombohedron which crystallises about the principal axis when the- 

 cellular conditions allow of material being deposited there, conse- 

 quent (following Minchin's account) on the adventitious occurrence 

 of another calcogenous cell on the gastral surface. That this crystal- 

 lographic interpretation is correct is indicated by the frequently 

 triangular section (fig. 7) of the fourth ray, the faces of the triangle 



PlG. 7. Three-spined quadriradiate spicules of A. cerebrum in perspective ; the- 

 interior of the distant rays are in one spicule indicated by shading. The- 

 fourth spicule is drawn from above ; it is not spined, but the apical ray shows 

 a distinctly triangular section. All x 260. 



being transverse to the rays of the triradiate, while a strong con- 

 firmation of the whole view is afforded by tlie thorns on the fourth 

 ray in Ascaltis cerebrum (fig. 7). These, when viewed from above, 

 are always seen to lie accurately over the rays of the triradiate,*' 

 either as definite minute triradiate spicules, or as three rows of 

 thorns ; and therefore crystallise on the three faces of the rhombo- 

 hedron. The whole quadriradiate, with fourth ray and thorns,, 

 extinguishes simultaneously in four positions between the crossed 

 nicols. I have verified on G. coriacea and bianco, [and A. reticulum~\ 

 Ebner's observation, that the optic axis of these equiangular spicules 

 is perpendicular to their plane. 



I suggest that the ancestors of these sponges had a skeleton con- 

 sisting of organic rods, partially impregnated with carbonate of lime; 

 that these *rods united in a triradiate grouping and that whether 

 for cementation or merely for strength the calcareous secretion in- 

 creased to the point of crystallisation. The original triradiate form 

 was due to an instinct, apparently acquired by the skeletogenoua 



with uniaxial optic picture, being vertical to the plane of the spicule, interpreta- 

 tion of these figures is not obvious. 



Glycerin commonly first decalcifies the axial parts, presumably because the liquid 

 enters most freely where there is greatest admixture of organic matter. Canada 

 balsam (except where the sponge bas been killed directly in chloroform) f rst 

 attacks the surface, presumably because the more permeable regions are pro- 

 tected by consolidated balsam. J*% 23, 1898.] 



* I have once or twice recorded a twist of 5 or 10; the thorns still beirg 

 inclined at 120 to each other. This, if established, would form another resem- 

 blance with the crystalline forms of dolomite. 



