590 THE SPIRAL SHELLS [ch. 



delicate structures endure; in dead shells, such as we are much 

 more famihar with, every trace of them is broken and rubbed 

 away. The growth of these long needles is explained (as we have 

 already briefly mentioned, on p. 440) by the phenomenon which 

 Lehmann calls orientirte Adsorption — the tendency for a crystalUne 

 structure to grow by accretion, not necessarily in the outward form 

 of a "crystal," but continuing in any direction or orientation 

 which has once been impressed upon it : in this case the spicular 

 growth is simply in direct continuation of the radial symmetry 

 of the micro-crystalhne elements of the shell-wall. Over the 

 surface of the shell the radiating spicules tend to occur in a 

 hexagonal pattern, symmetrically grouped around the pores which 

 perforate the shell. Rhumbler has suggested that this arrange- 

 ment is due to difEusion-currents, forming little eddies about the 

 base of the pseudopodia issuing from the pores: the idea being 

 borrowed from Benard, to whom is due the discovery of this type 

 or order of vortices*. In one of Benard's experiments a thin 

 layer of parafiin is strewn with particles of graphite, then warmed 

 to melting, whereupon each httle soMd granule becomes the centre 

 of a vortex ; by the interaction of these vortices the particles tend 

 to be repelled to equal distances from one another, and in the 

 end they are found to be arranged in a hexagonal pattern f. The 

 analogy is plain between this experiment and those diffusion 

 experiments by which Leduc produces his beautiful hexagonal 

 systems of artificial cells, with which we have dealt in a previous 

 chapter (p. 320). 



But let us come back to the shell itself, and consider particu- 

 larly its spiral form. That the shell in the Foraminifera should 

 tend towards a spiral form need not surprise us ; for we have 

 learned that one of the fundamental conditions of the production 

 of a concrete spiral is just precisely what we have here, namely 

 the gradual development of a structure by means of successive 

 increments superadded to its exterior, which then form part, 

 successively, of a permanent and rigid structure. This condition 



* Benard, H , Les tourbillons cellulaires.. Ann. de Chim'e (8), xxiv. 1901. Cf 

 also the pattern of cilia on an Infusorian, as figured by BiitschH in Bronn's 

 Protozoa, m, p. 1281, 1887. 



■j- A similar hexagonal pattern is obtained by the mutual repulsion of floating 

 magnets in Mr R. W. AVood's experiments, Phil. Mag. XLVi, pp. 162-164, 1898. 



