444 ON CONCRETIONS, SPICULES, ETC. [ch. 



third*. They are seldom in a plane, but are usually inclined to 

 one another in a solid, trihedral angle, not easy of precise measure- 

 ment under the microscope. The three rays are very often 

 supplemented by a fourth, which is set tetrahedrally, making, that 

 is to say, coequal 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 same 

 writer (but the fact, though it would seem easy to test, is still 

 disputed) there is no organic matter in the spicule, either in the 

 form of an axial filament or otherwise, and the appearance of 

 stratification, 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 ; more- 

 over it is a single crystal and not a group of crystals, as is at once 

 seen by its behaviour in polarised light. But its axes are not 

 crystalline axes, and its form neither agrees with, nor in any way 

 resembles, any one of the many polymorphic forms in which 

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

 out of a solid crj^stal ; 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 siirrounding cells, or structural 

 vesicles of the sponge. 



We have already studied in an elementary way, but amply 

 for our present purpose, the manner in which three or more cells, 

 or bubbles, tend to meet together under the influence of surface- 

 tension, and also the outwardly similar phenomena which may be 

 brought about by a uniform distribution of mechanical pressure. 

 We have seen that when we confine ourselves to a plane assemblage 

 of such bodies, we find them meeting one another in threes ; that 

 in a section or plane projection of such an assemblage we see the 

 partition- walls meeting one another at equal angles of 120° ; that 

 when the bodies are uniform in size, the partitions are straight 

 lines, which combine to form regular hexagons ; and that when 



* For very numerous illustrations of the triradiate and quadriradiate spicules 

 of the calcareous sponges, see {int. al.), papers by Dendy [Q. J. M. 8. xxxv, 1893), 

 Minchin [P. Z. S. 1904), Jenkin (P. Z. S. 1908), etc. 



