IX] OF THE NASSELLARIAN SKELETON 473 



(I think we may go so far as to say tnust have been formed) at 

 the interfaces of a little tetrahedral group of cells, the four equal 

 cells o the tetrahedron being in this particular case supplemented 

 by a little one in the centre of the system. We see, precisely as 

 in the internal boundary-system of an artificial group of four 

 soap-bubbles, the plane surfaces of contact, six in number ; the 

 relation to one another of each triple set of interfacial planes, 

 meeting one another at equal angles of 120'^ ; and finally the 

 relation of the four lines or edges of triple contact, which tend 

 (but for the little central vesicle) to meet at co-equal solid angles 

 in the centre of the system, all as we have described on p. 318. 

 In short, each triple- walled re-entrant angle of the little shell has 

 essentially the configuration (or a part thereof) of what we have 

 called a "Maraldi pyramid" in our account of the architecture of 

 the honeycomb, on p. 329*. 



There are still two or three remarkable or peculiar features in 

 this all but mathematically perfect shell, and they are in part easy 

 and in part they seem more difficult of interpretation. 



We notice that the amount of solid matter deposited in the 

 plane interfacial boundaries is greatly increased at the outer 

 margin of each boundary wall, where it merges or coincides with 

 the superficial furrow which separates the free, spherical surfaces 

 of the bubbles from one another ; and we may sometimes find that, 

 along these edges, the skeleton remains complete and strong, 

 while it shows signs of imperfect development or of breaking 

 away over great part of the rest of the interfacial surfaces. In 

 this there is nothing anomalous, for we have already recognised 

 that it is at the edges or margins of the interfacial partition -walls 

 that the manifestation of surface-energy will tend to reach its 

 maximum. And just as we have seen that, in certain of our 

 ''multicellular" spherical Radiol arians, it is at the superficial 



* Apart from the fact that the apex of each pyramid is interrupted, or truncated, 

 by the presence of the little central cell, it is also possible that the soUd angles 

 are not precisely equivalent to those of Maraldi's pyramids, owing to the fact that 

 there is a certain amount of distortion, or axial asymmetry, in the Nassellarian 

 system. In other words (to judge from Haeckel's figm'es), the tetrahedral symmetry 

 in Nassellaria is not absolutely regular, but has a main axis about which three of 

 the trihedral pyramids are symmetrical, the fourth having its solid angle somewhat 

 diminished. 



