460 



ON CONCRETIONS, SPICULES, 



[CH. 



to be developed as free and isolated formations or deposits, 

 precipitated in the colloid matrix, with no relation of form to 

 the cellular or vesicular boundaries. These simple acicular spicules 

 occupy a definite position in the organism. Sometimes, as for 

 instance among the fresh- water Hehozoa (e.g. Raphidiophrys), they 

 lie on the outer surface of the organism, and not infrequently 

 (when the spicules are few in number) they tend to collect round 

 the bases of the pseudopodia, or around the large radiating 

 spicules, or axial rays, in the cases where these latter are present. 

 When the spicules are thus localised around some prominent centre, 

 they tend to take up a position of symmetry in regard to it ; instead 

 of forming a tangled or felted layer, they come to lie side by side, 

 in a radiating cluster round the focus. In other cases (as for 

 instance in the well-known Radiolarian Aulacantha scolymantha) 

 the felted layer of aciculae lies at some depth below the surface, 

 forming a sphere concentric with the entire spherical organism. 

 In either case, whether the layer of spicules be deep or be super- 

 ficial, it tends to mark a "surface of discontinuity," a meeting 

 place between two distinct layers of protoplasm or between the 

 protoplasm and the w^ater around ; and it is obvious that, in either 

 case, there are manifestations of surface-energy at the boundary, 

 which cause the spicules to be retained there, and to take up their 

 position in its plane. The case is somewhat, though not directly, 

 I , analogous to that of a cirrus cloud, 



which marks the place of a surface 

 of discontinuity in a stratified at- 

 mosphere. 



We have, then, to enquire what 

 are the conditions which shall, apart 

 from gravity, confine an extraneous 

 body to a surface-film ; and we may 

 do this very simply, by considering 

 the surface-energy of the entire 

 system. In Fig. 218 we have two 

 fluids in contact with one another 

 (let us call them water and proto- 

 plasm), and a body (b) which may 

 be immersed in either, or mav be restricted to the boundary 



Fiff. 218. 



