The Skeleton and Classification of Calcareous Sponges. 69 



perpendicular to the plane of the spicule (fig. 10, o.a.). The curva- 

 ture of rajs has been adduced by Schulze (11^ as an argument against 



FIG. 9. Alate spicule of L. LielerkilJmii ( x 150). 



i 



FIG. 10. Diagram of profile view of the spicuie snown in fig. 9. The curvature is 

 from measurement; the angle made by oa, the optic axis, is copied from 

 Ebner's figure of a similar spicule, in S. elegans. 



crystalline structure ; but the form shown in fig. 9 is the trace on a 

 cylindrical surface of three planes, which meet at equal angles in a 

 line forming an acute angle with the axis of the cylinder, to which, 

 one plane is radial, two of them, therefore, cut the cylinder in 

 ellipses, and one in a straight line. Investigation shows that the 

 angle agrees with that given by Sollas and Ebner for the optic 

 axis of a spicule of similar form.* The alate triradiate of Leucoso- 

 lenia, with curved rays, is therefore as accurately regular a skeleton- 

 crystal as the equiangular triradiate of Clathrina. Both follow the 

 same law, that the calcareous secretion is limited by the form of the 

 sponge to the surface of a cylinder, and crystallisation of the 

 secretion takes place in the radial planes of a rhombohedron which 

 are perpendicular to its three faces, the difference being only that in 

 Clathrina the optic axis of the rhombohedron is radial to the cylinder, 

 and in Leucosolenia it lies in a radial plane of the cylinder at an acute 

 angle with the axis.f 



* To fellow-naturalists my own method of investigation may prove helpful. 

 Draw the spicule (fig. 9) two or three inches long on a sheet of paper ; look at the 

 diagram with the eye where the line oa (fig. 10) would come, and curve ths paper 

 so that it forms part of a cylinder the axis of which is parallel with the long ray. 

 The diagram will then appear to be that of an equiangular spicule with straight ' 

 rays. 



f The fourth ray in Leucosolenia and allied sponges grows (perhaps most freely 

 at right angles to the optic axis ?) to an edge of the rhombohedron. It is often 

 curved, a character possibly resulting from the suspension of the formative cell 

 between it and the gastral surface. In L. LielerJcuhnii, as observed by Minchin 

 for L. complicata, its origin is on the unpaired ray. Unlike the apical ray 



