IX] 



AND SPICULAR SKELETONS 



699 



collect rouiul the bases of the [)seiidopo(lia, or around the larger 

 radiating spicules or axial rays in cases where these latter are 

 present. When the spicules are thus localised around some pro- 

 minent 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 superficial, it tends to mark a "surface of discontinuity," a 

 meeting place either between two distinct layers of protoplasm or 

 between the protoplasm and the water around; and it is evident 

 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 analogous to that 

 of a cirrus cloud, which marks a surface of discontinuity in a 

 stratified atmosphere. 



We have, then, to enquire w^hat are the conditions, apart from 

 gravity, which confine an extraneous body to a surface-film ; and 

 we may do this very simply, by con- 

 sidering the surface-energy of the entire 

 system. In Fig. 319 we have two fluids 

 in contact with one another (let us call W 

 them water and protoplasm), and a 

 body (6) which may be immersed in 

 either, or may be restricted to the 

 boundary between. We have here 

 three possible "interfacial contacts," 

 each with its own specific surface- 

 energy per unit of surface area: 

 namely, that between our particle and 

 the water (let us call it a), that between 

 the particle and the protoplasm (^), and 

 that between water and protoplasm (y). When the body lies in 

 the boundary of the two fluids, let us say half in one and half 

 in the other, the surface-energies concerned are equivalent to 

 (>S'/2) a 4- (^/2) ^; but we nmst also remember that, by the presence 



Fig. 319. 



