V] OF THE SUN-ANIMALCULES 427 



protoplasm, that is to say, from the more fluid surface-protoplasm 

 of the cell; and the latter begins to creep up the filament, just as 

 water would creep up the interior of a glass tube, or the sides of 

 a glass rod immersed in the liquid. It is the simple case of a balance 

 between three separate tensions: (1) that between the filament and 

 the adjacent protoplasm, (2) that between the filament and the 

 adjacent water, and (3) that between the water and the protoplasm. 

 Catting these tensions respectively T/^, T^^,, and T^^^,, equilibrium 

 will be attained when the angle of contact between the fluid 



T — T 



protoplasm and the filament is such that cos a = -^~r^ — ~' - It is 



evident in this case that the angle is a very small one. The precise 

 form of the curve is somewhat different from that which, under 

 ordinary circumstances, is assumed by a liquid which creeps up a 

 solid surface, as water in contact with air creeps up a surface of 

 glass; the -difference being due to the fact that here, owing to the 

 density of the protoplasm being all but identical wuth that of the 

 surrounding medium, the whole system is practically immune from 

 gravity. Under normal circumstances the curve is part of the 

 "elastic curve" by which that surface of revolution is generated 

 which we have called, after Plateau, the nodoid; but in the present 

 case it is apparently a catenary. Whatever curve it be, it obviously 

 forms a surface of revolution around the filament. 



Since this surface-tension is symmetrical around the filament, the 

 latter will be pulled equally in all directions; in other words the 

 filament will tend to be set normally to the surface of the sphere, 

 that is to say radiating directly outwards from the centre. If the 

 distance between two adjacent filaments be considerable, the curve 

 will simply meet the filament at the angle a already referred to; 

 but if they be sufficiently near together, we shall have a continuous 

 catenary curve forming a hanging loop between one filament and 

 the other. And when this is so, and the radial filaments are more 

 or less symmetrically interspaced, we may have a beautiful system 

 of honeycomb-like depressions over the surface of the organism, 

 each cell of the honeycomb having a strictly defined geometric 

 configuration (cf. p. 710). 



In the simpler Radiolaria. the spherical form of the entire organism 

 is equally well marked; and here, as also in the more complicated 



