94. _The Sarcodictyum._--The sarcodictyum may be defined as the extracalymmar network of exoplasm, and is a reticular covering which lies upon the outer surface of the gelatinous calymma. Internally, the sarcodictyum is in direct communication with the sarcoplegma, or the web of exoplasmic threads which ramifies in the gelatinous substance of the calymma; externally, on the other hand, the pseudopodia radiate freely from it; thus its relation to these is similar to that which the sarcomatrix bears to the roots of the sarcoplegma. Relations similar to those which have led to the separation of the primary from the secondary calymma, induce us to distinguish also a primary and secondary sarcodictyum. The original or _primary sarcodictyum_ ramifies over the surface of the original or primary calymma, and like this is of pre-eminent importance in the formation of the primary lattice-shell; if we regard the surface of the primary calymma as the indispensable foundation for the deposition of this latter, then the primary sarcodictyum furnishes the material from which it is developed: silex in the SPUMELLARIA and NASSELLARIA, a silicate of carbon in the PHÆODARIA, and acanthin in the ACANTHARIA. It may indeed be said that the primary lattice-shell of the Radiolaria arises by a direct chemical metamorphosis of the primary sarcodictyum, by a chemical precipitation of the dissolved skeletal material (silex, silicate, or acanthin), which was stored up in the exoplasm of the sarcodictyum. Hence a deduction from the special conformation of the former to that of the latter is permissible. The particular form of the primary lattice-sphere with its regular or irregular meshes is due to the corresponding form of the primary sarcodictyum; both regular and irregular forms of this {lxii}commonly occurring. The form of the _regular sarcodictyum_ with circular or regular polygonal, usually hexagonal, meshes is constantly maintained during the formation of the regular lattice-shells (_e.g._, Pl. 12, figs. 5-10; Pl. 52, figs. 8-20; Pl. 96, figs. 2-6; Pl. 113, figs. 1-6). The form of the _irregular sarcodictyum_, on the other hand, with irregular polygonal or roundish meshes, persists during the development of the irregular lattice-shells (_e.g._, Pls. 29, 70, 97, 106). All this is true also of the _secondary sarcodictyum_, or the exoplasmic network which ramifies over the surface of the secondary calymma. The secondary lattice-shells, which are deposited on the surface of the latter, retain the configuration of the secondary sarcodictyum, by the chemical metamorphosis of which they have originated; this is the case in many SPUMELLARIA which develop several concentric lattice-shells (Pl. 29), in some NASSELLARIA (Pl. 54, fig. 5), in the Phractopeltida among the ACANTHARIA (Pl. 133), and in the double-shelled PHÆODARIA, Cannosphærida, and part of the Coelodendrida and Coelographida (Pls. 112, 121, 128). In those Radiolaria which form no