The #Discoidea# on the contrary must be derived from the #Sphæroidea# by shortening in one axis; here therefore the fenestrated shell becomes more or less lenticular or iscoidal (often with radial spines or arms in the equatorial plane, on the circular margin). The #Larcoidea#, the fourth section, differ from the three foregoing sections by the different growth of the shell in three different dimensions of space; therefore here the fenestrated shell becomes "lentelliptical," or a "triaxial ellipsoid," its length, breadth, and height being different. _The Skeleton_ consists in all SPUMELLARIA either of pure _silica_ or of a peculiar silicate. The siliceous bars and beams constituting it are invariably _solid_, as also in the NASSELLARIA, never hollow, as in the PHÆODARIA. Never is the skeleton composed of acanthin, as in all ACANTHARIA. Whilst in the first order of SPUMELLARIA, #Collodaria#, the form of the spicula, or the scattered needles, composing the skeleton, is very simple, _never latticed_, in the second order, the #Sphærellaria#, it is constantly latticed or fenestrated, often also spongy. The geometrical fundamental form of the lattice-shell in the #Sphærellaria# is originally spherical (homaxon), as preserved in all #Sphæroidea#; in the #Prunoidea# and #Discoidea# it becomes monaxon, with one single axis (prolonged in the former, shortened in the latter); in the #Larcoidea# it becomes triaxon, by different growth in three principal axes, perpendicular one to another. The further development of radial parts of the skeleton in these three axes is very important for the "promorphology" of the Radiolaria. _The Malacoma_, or the whole soft body of the SPUMELLARIA as opposed to the skeleton, exhibits some differences of structure in two different groups, which were separated formerly (1862) as Monocyttaria and Polycyttaria, corresponding to the "Radiolaria monozoa and polyzoa" of Johannes Müller (1858). The #Monocyttaria# (or the Spumellaria solitaria) live isolated as single cells--like {8}all other Radiolaria--and are never aggregated in colonies; the calymma includes one single central capsule, and this again one central nucleus, which does not become divided until full maturity. The #Polycyttaria# on the contrary (or the Spumellaria socialia) live aggregated in large colonies; the calymma includes a variable number of associated central capsules and each of these commonly one central oil-globule, whilst the original simple nucleus commonly becomes very early divided into numerous small nuclei.