The Skeleton and Classification of Calcareous Sponges. 63 



as a whole, to the action of crystallisation." If I understand my f riead 

 correctly, I differ from him essentially. I regard the spicules of 

 calcareous sponges as being skeleton crystals of calcite. The direction 

 of the optic axis is fixed (probably by the conditions of pressure and 

 tension prevailing at the moment) when the spicule first makes its 

 appearance. The formed material from which the calcite crystallises 

 is commonly limited by biological conditions to a narrow interval 

 between two similar surfaces. These surfaces may have the form 

 of a comparatively wide cylinder, as in the case of Clathrina clatlirus 

 or Leucosolenia Lieberkiihnii, a narrow cylinder, as in the case of the 

 stalk of Guanclia blanca, or a more complex irregular form, as in the 

 afferent or radial canals of the Heteroccela. I suggest that the 

 triradiate spicule is in all cases the skeleton of that region of a 

 hexagonal prism, with the given optic axis, which is enclosed by the 

 two surfaces which limit the form'ttion of material. 



It is impossible to profess, in a limited space, or indeed with the 

 limits of our available knowledge, to give anything approaching a 

 complete harmony of all known forms of spicules with the foregoing 

 hypothesis. Treating of the simplest, the regular triradiates of 

 Clathrina clathrus and Guanclia coriacea, I have been led to the 

 above conclusion primarily by observation of deformed spicules; of 

 which I sketched all examples met with during many months 

 examination of fresh preparations at Naples, and for which I have 



FlG. 1. Spicules of Guancha (Leucosolenia} coriacea (Mx>nt.) from Naples, x 120. 

 a is normal, some hundreds of this type occurring for one of all the others. 

 In this species and C. clathrus 26 or 27 deformed spicules were recorded. 

 15 belonging to type 5, 6 to type c, 3 to type d, 1 to type e, and 1 to type/ 1 . 

 In addition to these, a drawing was lost of a spicule of type b, but with the 

 ray bent a second time to its original course, and Ebner gives a drawing of a 

 plane quadriradiate X-shaped spicule (in C. clathrus}, the four angles being 

 alternately 120 and 60. No other deformed spicule was observed in these 

 species. [Ebner also draws a form resembling /, but with the two flexions 

 symmetrical and in opposite senses.] 



since carefully searched my permanent preparations. Drawings 

 of all types met with, among some thirty examples, are given in 

 fig. 1. It will be seen that they are all expressed in one statement, 

 perpendicular to them. I shall use the term "triradiate" where necessary with 

 the same meaning. 



F 2 



