130. _Dictyosis or Lattice Formation of the Skeleton._--In the great majority of Radiolaria the dictyosis or formation of lattice-work, and especially the formation of a variously-shaped "lattice-shell," plays such an important part that the whole class has long been popularly known in Germany by the name "lattice animalcules" ("Gitterthierchen" or "Gitterlinge") (_Protista dictyota_). The old name Polycystina also (1838), although referring only to the SPUMELLARIA and NASSELLARIA, is derived from the lattice-work of the siliceous skeleton. The extremely various forms in which this is manifested furnish the means of distinguishing species. The specific conformation of the skeletal lattice-work is usually caused by the special disposition of the sarcodictyum (§ 94), whose exoplasmatic threads become silicified or (in the ACANTHARIA) converted into bars of acanthin. In many cases, however, the form of the lattice is mainly dependent upon the situation and form of the radial spines or of special processes from them. With respect to their origin, two varieties of lattice may be distinguished--simultaneous and successive. _Simultaneous dictyosis_ occurs especially in the simple lattice-shells of the #Sphærellaria# and PHÆODARIA, where, at a given moment ("dictyotic moment") the _whole_ lattice of the shell is excreted on the surface of the calymma. _Successive dictyosis_, on the other hand, is found more particularly in the lattice-shells of the ACANTHARIA (and in the concentric cortical shells of many #Sphærellaria#), which develop from the separate lattice-plates formed by the apophyses of the radial spines, and hence not at the same moment. The lattice-shells of the #Cyrtellaria#, which gradually grow out from a sagittal ring or a basal tripod, arise by successive dictyosis.