IX] AND SPIOULAR SKELETONS 707 



(2) in the form of a crust, developed either on the outer surface of 

 the organism or in relation to one or more of the internal surfaces 

 which separate its concentric layers or its component vesicles. 

 Not infrequently, this superficial skeleton comes to constitute a 

 spherical shell, or a system of concentric spheres. 



We have already seen that a great part of the body of the 

 Radiolarian, and especially that outer portion to which Haeckel 

 has given the name of the "calymma," is built up of a mass of 

 "vesicles," forming a sort of stiff froth, and equivalent in the 

 physical though not necessarily in the biological sense to "cells," 

 inasmuch as the little vesicles have their own well-defined boun- 

 daries, and their own surface phenomena. In short, all that we 

 have said of cell-surfaces and cell-conformations in our discussion 

 of cells and of tissues will apply in like manner, and under appro- 

 priate conditions, to these. In certain cases, even in so common 

 and so simple a one as the vacuolated substance of an Actino- 

 sphaerium, we may see a close resemblance, or formal analogy, to 

 a cellular or parenchymatous tissue in the close-packed arrangement 

 and consequent configuration of these vesicles, and even at times 

 in a shght membranous hardening of their walls. Leidy has figured * 

 some curious httle bodies hke small masses of consoUdated froth, 

 which seem to be nothing else than the dead and empty husks, or 

 filmy skeletons, of Actinosphaerium ; and Carnoyf has demon- 

 strated in certain cell-nuclei an all but precisely similar framework, 

 of extreme minuteness and tenuity, formed by adsorption or partial 

 sohdification of interstitial matter in a close-packed system of 

 alveoh (Fig. 321). In short, we are again dealing or about to deal 

 with a network or basketwork, whose meshes correspond to the 

 boundary fines between associated cells or vesicles. It is just in 

 those boimdary walls or films, still more in their edges or at their 

 corners, that surface-energy will be concentrated and adsorption 

 will be hard at work; and the whole arrangement will follow, or 

 tend to follow, the rules of areas minimae — the partition-walls 

 meeting at co-equal angles, three by three in an edge, and their 

 edges meeting four by four in a corner. 



Let us suppose the outer surface of our Radiolarian to be covered 



* J. Leidy, Fresh-water Rhizopods of North America, 1879, p. 262, pi. xli, figs. 11,12. 

 t Carnoy, Biologie Cellulaire, p. 244, fig. 108; cf. Dreyer, op. cit. 1892, fig. 185. 



