185. _Ascent of the Nassellaria from the Cyrtoidea._--The monophyletic hypothesis (No. 3, p. 894) which regards the latticed cephalis as the common point of origin of all the skeletons of the NASSELLARIA, starts from the simplest forms of the #Cyrtoidea#, that is, from the Cyrtocalpida or eradial Monocyrtida (_Archicorida_, _Archicapsida_, Pls. 51, 52, 98). All #Cyrtoidea# and #Botryodea# may be regarded as divergent forms of these monothalamous #Cyrtoidea#; the polythalamous simply by the addition of fresh joints at the basal pole, the triradiate and multiradiate by the development of three or more apophyses. The origin of the sagittal ring (which presents every stage of development and degeneration in the #Cyrtoidea#) may be regarded as a mechanical thickening of the latticed plate in the sagittal circumference of the cephalis. By stronger { cxx}development of this ring and coincident sagittal constriction of the cephalis the order #Spyroidea# may be derived from the #Cyrtoidea#. On the other hand, the #Plectellaria#, which possess no cephalis, and indeed no complete lattice-shell whatever, may be derived from the Monocyrtida by the assumption of a degeneration of this structure; the sagittal ring having been preserved in the #Stephoidea#, and the tripod of the Tripocalpida in the #Plectoidea#. Although such a monophyletic derivation of the NASSELLARIA from the Cyrtocalpida is possible, and though here, too, the Cortinida play an important part as connecting links, this hypothesis has less internal probability than that of the derivation from the #Stephoidea# (§ 184) or #Plectoidea# (§ 183).
186. _Genealogical Tree of the Plectoidea._--The order #Plectoidea# includes those NASSELLARIA whose rudimentary skeleton does not contain the characteristic sagittal ring of the #Stephoidea#, but consists of several (at least three) radial spines, which proceed from a point in the centre of the porochora. The branches of these radial spines remain free in the Plagonida, whilst in the Plectanida they unite with each other to form a loose meshwork (not, however, a complete lattice-shell). The number and arrangement of the radial spines, which serve for generic distinctions, are the same in both families, so that each genus of the Plectanida has arisen from a corresponding genus of the Plagonida. The simplest Plagonida, which possess a basal tripod (_Triplagia_ or _Plagiacantha_ with three rays, _Tetraplagia_ with four rays) are probably to be regarded as forming the common origin of the whole order. These agree with certain three- and four-rayed skeletal pieces of the #Beloidea# (Thalassosphærida and Sphærozoida); and also the four and six-rayed twinned pieces of the latter (spicula bigemina and trigemina) repeat in the same fashion the skeleton of the former (_Plagonidium_, _Plagonium_). This similarity, however, is a mere analogy and possesses no phylogenetic significance. On the other hand, certain Plagonida (_Plagoniscus_, _Plagiocarpa_), and the corresponding genera of Plectanida (_Plectaniscus_, _Periplecta_) seem to have important phylogenetic relations to certain #Stephoidea# (_Cortina_, _Cortiniscus_, &c.); the sagittal ring of the latter having perhaps arisen by the vertical apical spine of the former having been connected with their horizontal basal rod by two ventral apophyses growing out opposite to each other (compare pp. 902, 914, _Plagiocarpa procortina_, Pl. 91, fig. 5). In this case the Plectanida would belong to the simplest stem-forms of the NASSELLARIA.