178. _Phylogeny of the Nassellaria._--The legion NASSELLARIA or MONOPYLEA is so clearly characterised by the peculiar porochora, which closes the osculum at the oral pole of the monaxon central capsule, and by the podoconus connected with it, that there can be no doubt that phylogenetically it represents an independent stem (§ 8). This stem is only connected at its base by means of _Cystidium_ and _Nassella_ with _Actissa_ and _Thalassicolla_, the stem-forms of the SPUMELLARIA. This stem is monophyletic, inasmuch as all its members may be derived without violence from the skeletonless Nassellida (_Nassella_, _Cystidium_, p. 896, Pl. 91, fig. 1). 179. _Origin of the Nassellaria._--The Nassellida (p. 896), which may naturally be considered as the common stem-group of the NASSELLARIA, are most nearly related among other Radiolaria to the Thalassicollida, and in both these skeletonless families the simplest forms, _Cystidium_ and _Actissa_ correspond; on the other hand, those which have arisen from them by the formation of alveoles in the calymma (_Nassella_ and _Thalassicolla_) also correspond. The origin of the simplest Nassellida from these primitive Thalassicollida may be explained by supposing that the numerous (formerly evenly distributed) pores of the capsule membrane became obliterated in the upper (apical) half of the central capsule, whilst in the lower (basal) half they became correspondingly more strongly developed; hence the porochora was formed at the oral pole of the vertical main axis, and a differentiation of the endoplasm proceeding from this gave rise to the characteristic podoconus. Both these organs still at present exhibit very various degrees of progressive development. {cxvii}180. _Hypothetical Genealogical Tree of the Nassellaria._