133. _Dictyosis of the Nassellaria._--The siliceous lattice-structures of the NASSELLARIA are formed on the whole like those of the SPUMELLARIA, with which they were formerly united under the name "Polycystina." In this group also there may be distinguished as two main forms the regular and irregular. In the NASSELLARIA the regular lattice-structures generally exhibit hexagonal or circular meshes, whilst the irregular are either polygonal or roundish; the irregular forms are, however, much more abundant than the {lxxxviii}regular, and a further distinction from the SPUMELLARIA consists in the fact that the primary skeletal elements, from which the lattice is secondarily developed, exercise a predominant influence upon their form. These primary elements in the majority of the NASSELLARIA are to be seen in two morphologically most important structures:--first, the _primary sagittal ring_, which embraces the central capsule in the median plane (§ 124); and secondly, the _basal tripod_ (§ 125), whose three diverging rays proceed from the base of the central capsule, whilst commonly a fourth vertical ray supports the dorsal side of latter (compare Pls. 81-91, p. 892). In the majority of the NASSELLARIA these two primary elements appear in combination, whilst in others only one of them is recognisable. In addition there occur numerous monaxon lattice-shells in which neither of these elements can be recognised, but a simple ovoid lattice-shell (cephalis) alone forms the whole skeleton or its primary part (Pl. 51, fig. 13; Pl. 98, fig. 13). The great difficulty in the morphological interpretation and phylogenetic derivation of the NASSELLARIAN skeleton lies in the fact that each of these three elements--the primary sagittal ring, the basal tripod, and the latticed cephalis--may form the whole skeleton by itself or be combined with one or both of the others (p. 893). Even nearly related or at all events very similar forms may differ very greatly in this respect. With regard to the manifold forms of their dictyosis it follows that it is partly dependent upon one of the two first elements, partly independent. In the #Plectellaria# (or those NASSELLARIA which do not possess a complete lattice-shell) the lattice-work is usually irregular and arises by union of the ramifications, which proceed either from the primary sagittal ring (Pls. 81, 82, 92-94) or from the basal tripod (Pl. 91). In the #Cyrtellaria# (or NASSELLARIA with a complete lattice-shell, Pls. 51-80), on the other hand, the lattice-work is sometimes regular, sometimes irregular, being often very different in the different joints of a segmented shell (Pl. 72); a great part of it arises independently of the two chief morphological elements, and develops according to laws similar to those which regulate the dictyosis of the SPUMELLARIA.