said) that the tensions of the three partitions draw inwards the outer walls of the system, till at each point of triple contact (P) we tend to get a triradiate, equiangular junction. But if we introduce another bubble into the centre of the system (Fig. 230), then, as Plateau shewed, the tensions of its walls and those of the three partitions by which it is now suspended, again balance one another, and the central bubble appears (in plane projection) as a curvilinear, equilateral triangle. We have only got to convert this plane diagram into that of a tetrahedral solid to obtain almost precisely the configuration which we are seeking to explain. Now we observe that, so far as our figure of Callimitra informs us, this is just the shape of the little bubble which occupies the centre of the tetrahedral system in that Radiolarian skeleton. And I conceive, accordingly, that the entire organism was not limited to the four cells or vesicles (together with the little central {477} fifth) which we have hitherto been imagining, but there must have been an outer tetrahedral system, enclosing the cells which fabricated 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 fabric81.
In this hypothetical case, the edges of the skeletal system would
be circular arcs, meeting one another at an angle of 120ô¯, or, in the
solid pyramid, of 109ô¯: 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 slightly drawn out into projecting
cusps (cf. Fig. 230, B). There is
no 
In the foregoing examples of Radiolaria,