froth, and equivalent in the physical sense (though not nec­es­sar­i­ly in the bio­log­i­cal sense) to “cells,” in­as­much as the little vesicles have their own well-defined boun­daries, 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 ap­prop­ri­ate con­di­tions, to these. In cer­tain cases, even in {468} 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[481] some curious little bodies, like small masses of con­sol­i­dated froth, which seem to be nothing else than the dead and empty husks, or filmy skeletons, of Ac­ti­no­sphae­rium. And Carnoy[482] 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. Aulonia). Such a conformation is {469} extremely common, and among its many variants may be found cases in which (e.g. Actinomma), the vesicles have

Fig. 221. Aulonia hexagona, Hkl.

Fig. 222. Actinomma arcadophorum, Hkl.

been less regular in size, and some in which the hexagonal meshwork has been developed not only on one outer surface, but at successive {470} surfaces, producing a system of concentric spheres. If the siliceous material be not limited to the linear junctions of the cells, but spread over a portion of the outer spherical surfaces or caps, then we shall have the condition represented in Fig. [223] (Ethmosphaera), where the shell appears perforated by circular instead of hexagonal apertures, and the circular pores are set on slight spheroidal eminences; and, interconnected with such types as this, we have others in which the accumulating pellicles of skeletal matter have extended from the edges into the substance of the boundary walls

Fig. 223. Ethmosphaera conosiphonia, Hkl.Fig. 224. Por­tions of shells of two “spe­cies” of Ce­no­sphae­ra: up­per fig­ure, C. fav­o­sa, low­er, C. ves­pa­ria, Hkl.

and have so produced a system of films, normal to the surface of the sphere, constituting a very perfect honeycomb, as in Cenosphaera favosa and vesparia[483].