468 



ON CONCRETIONS, SPICULES, ETC. 



[CH. 



SO common and simple a one as the vacuolated substance of an 

 Actinosphaerium, we may see a very close resemblance, or formal 

 analogy, to an ordinary cellular or " parenchymatous" tissue, in the 

 close-packed arrangement and consequent configuration of these 

 vesicles, and even at times in a slight membranous hardening of 



their walls. Leidy has figured* 

 some curious little bodies, like 

 small masses of consohdated 

 froth, which seem to be nothing 

 else than the dead and empty 

 husks, or filmy skeletons, of 

 Actinosphaerium. And Carnoyj 

 has demonstrated in certain 

 cell-nuclei an all but precisely 

 similar framework, of extreme 

 delicacy and minuteness, as the 

 result of partial solidification 

 of interstitial matter in a close- 

 packed system of alveoli (Fig. 

 220). 



Let us now suppose that, 

 in our Radiolarian, the outer 

 surface of the animal is covered by a layer of froth-like vesicles, 

 uniform or nearly so in size. We know that their tensions will 

 tend to conform them into a " honeycomb," or regular meshwork 

 of hexagons, and that the free end of each hexagonal prism will 

 be a little spherical cap. Suppose now that it be at the outer 

 surface of the protoplasm (that namely which is in contact with 

 the surrounding sea-water), that the siliceous particles have a 

 tendency to be secreted or adsorbed; it will at once follow that 

 they will show a tendency to aggregate in the grooves which 

 separate the vesicles, and the result will be the development of 

 a most delicate sphere composed of tiny rods arranged in a regular 

 hexagonal network (e.g. Aulonid). Such a conformation is 



Fig. 220. 



"Reticulum plasmatique. 

 (After Carnoy.) 



* Leidy, J., Fresh-water Bhizopods of N. America, 1879, p. 262, pi. xU, 

 figs. 11, 12. 



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

 fis;. 185. 



