IX] AND SPICULAR SKELETONS ' 477 



fifth) which we have hitherto been imagining, but there must have 

 been an outer tetrahedral system, enclosing the cells which fabri- 

 cated the skeleton, just as these latter enclosed, and deformed, 

 the little bubble in the centre of all. We have only to suppose 

 that this hypothetical tetrahedral series, forming the outer layer or 

 surface of the whole system, was for some chemico-physical reason 

 incapable of secreting at its interfacial contacts a skeletal fabric*. 

 In this hypothetical case, the edges of the skeletal system would 

 be circular arcs, meeting one another at an angle of 120°, or, in the 

 solid pyramid, of 109° : and this latter is very nearly the condition 

 which our little skeleton actually displays. But we observe in 

 Fig. 227 that, in the immediate neighbourhood of the tetrahedral 

 angle, the circular arcs are shghtly drawn out into projecting 

 cusps (cf. Fig. 230, B). There is no S-shaped curvature of the 

 tetrahedral edges as a whole, but a very slight one, a very shght 

 change of curvature; close to the apex. This, I conceive, is 

 nothing more than what, in a material system, we are bound to 

 have, to represent a "surface of continuity." It is a phenomenon 

 precisely analogous to Plateau's "bourrelet," which we have 

 already seen to be a constant feature of all cellular systems. 

 rounding off the sharp angular contacts by which (in our more 

 elementary treatment) we expect one film to make its junction 

 with another f. 



In the foregoing examples of Radiolaria, the symmetry which 

 the organism displays would seem to be identical with that 

 symmetry of forces which is due to the assemblage of surface- 

 tensions in the whole system ; this symmetry being displayed, in 

 one class of cases, in a complex spherical mass of froth, and in 



* We need not go so far as to suppose that the external layer of cells wholly 

 lacked the power of secreting a skeleton. In many of the NasseUariae figured by 

 Haeckel (for there are many variant forms or species besides that represented here), 

 the skeleton of the partition-walls is very shghtly and scantily developed. In 

 such a case, if we imagine its few and scanty strands to be broken away, the central 

 tetrahedral figure would be set free, and would have all the appearance of a complete 

 and independent structure. 



f The "bourrelet" is not only, as Plateau expresses it, a "surface of continuitv," 

 but we also recognise that it tends (so far as material is available for its production) 

 to further lessen the free surface-area. On its relation to vapour-pressure and to 

 the stability of foam, see FitzGerald's interesting note in Nature, Feb. 1, 1894 

 {Works, p. 309). 



