4. _Osculosa or Merotrypasta._--The subclass Osculosa or Merotrypasta includes the two legions MONOPYLEA or NASSELLARIA, and CANNOPYLEA or PHÆODARIA, which agree in the following constant and important characters:--(1) The Central Capsule is originally monaxon (ovoid or spheroidal) and retains this ground-form in most of the species. (2) The _Membrane_ of the central capsule possesses a single large principal aperture (osculum) at the basal pole of the vertical main axis. (3) The _Pseudopodia_ radiate from a stream of sarcode which passes out from the central capsule only on one side, namely, through the principal aperture. (4) The _Equilibrium_ of the floating body is {iii}monostatic or unistable, since the two poles of the principal axis are always more or less different from each other. (5) The _Ground-forms_ of the skeleton are, therefore, for the most part grammotypic (centraxon) or zygotypic (centroplan), rarely spherotypic. The two legions of the Osculosa are distinguished chiefly by the principal opening (osculum) being closed by a porous plate (porochora with its podoconus) in the NASSELLARIA (or MONOPYLEA), and by a radiate cover (operculum with its astropyle) in the PHÆODARIA (or CANNOPYLEA). 5. _The four Legions of Radiolaria._--The four principal groups of Radiolaria, to which we have given the name "legions," are natural units, since the most important peculiarities in the structure of the central capsule are quite constant within the limits of the same legion, and since all the forms in the same legion may be traced without violence to the same phylogenetic stem. The four legions are, however, related to each other, in so far as they all exhibit those characters which distinguish the Radiolaria from other Protista. The two which compose the Porulosa (§ 3) seem somewhat more nearly related to each other than to the two which make up the Osculosa (§ 4). When, however, the attempt is made to bring them all into a phylogenetic relationship, it undoubtedly appears that the SPUMELLARIA (or PERIPYLEA) are the primitive stem, out of which the other three have been developed as independent branches. All three have been derived, probably independently, from the most ancient stem-form of the SPUMELLARIA, the spherical _Actissa_.