IX] OF'THE SKELETON OF SPONGES 679 



assumption that they develop in association (of some sort or another) 

 with the lines of junction, or boundary-edges, of contiguous cells. 

 They include the triradiate spicules of the calcareous sponges, the 

 quadriradiate or " tetractinelHd " spicules which occur sometimes 

 in the same group but more characteristically in certain siHceous 

 sponges known as the TetractineUidae, and perhaps (though these 

 last are somewhat harder to understand) the six-rayed spicules of 

 the HexactinelHds. We shall come later on to more compUcated 

 skeletons of the same type among the Radiolaria. 



The spicules of the calcareous sponges are commonly triradiate, 

 and the three radii are usually inclined to one another at nearly 

 equal angles; in certain cases, two of the three rays are nearly in 

 a straight line, and at right angles to the third *. They are not always 

 in a plane, but are often inchned to one another in a trihedral 

 angle, not easy of precise measurement under the microscope. The 

 three rays are often supplemented by a fourth, which is set tetra- 

 hedrally, making nearly co-equal angles with the other three. The 

 calcareous spicule consists mainly of carbonate of lime in the form 

 of calcite, with (according to von Ebner) some admixture of soda 

 and magnesia, of sulphates and of water. According to the sarne 

 writer there is no organic matter in the spicule, either in the form 

 of an axial filament or otherwise, and the appearance of stratifica- 

 tion, often simulating the presence of an axial fibre, is due to "mixed 

 crystallisation" of the various constituents. The spicule is a true 

 crystal, and therefore its existence and its form are primarily due 

 to the molecular forces of crystallisation; moreover it is a single 

 crystal and not a group of crystals, as is seen by its behaviour in 

 polarised fight. But its axes are not crystalline axes, its angles are 

 variable and indefinite, and its form neither agrees with, nor in any 

 way resembles, any one of the countless, polymorphic forms in 

 which calcite is capable of crystallising. It is as though it were 

 carved out of a solid crystal; it is, in fact, a crystal under restraint, 

 a crystal growing, as it were, in an artificial mould, and this mould 

 is constituted by the surrounding cells or structural vesicles of the 

 sponge. 



* For very numerous illustrations of the triradiate and quadriradiate spicules 

 of the calcareous sponges, see (int. at.), papers by Dendy {Q.J. M.S. xxxv, 1893), 

 Minchin {P.Z.S. 1904), Jenkin {P.Z.S. 1908), etc. 



