{ cviii}162. _Collodaria and Sphærellaria._--Whilst in all SPUMELLARIA the malacoma agrees in possessing the characteristic features of the legion, and thus justifies its derivation monophyletically from the common stem-form _Actissa_, the different forms of skeleton, on the other hand, cannot all be referred to the same fundamental form. More especially the _spherical lattice-shell_, from which all the numerous skeletal forms of the #Sphærellaria# may be derived, cannot have arisen from the incomplete Beloid skeleton which characterises the #Beloidea# among the #Collodaria#. It is probable rather that the formation of the skeleton has taken place independently in those two groups of SPUMELLARIA. From the skeletonless #Colloidea#, as the common stem-group of the SPUMELLARIA, two different main groups have diverged, on the one hand the #Beloidea#, whose skeleton consists of separate spicules scattered in the extracapsulum, and on the other hand, the #Sphærellaria#, which have formed a simple lattice-sphere around the central capsule; from this the manifold forms of the remaining SPUMELLARIA may be derived.
163. _Descent of the Sphærellaria._--The extensive order #Sphærellaria#, which includes all SPUMELLARIA with a complete lattice-shell, develops an extraordinary variety of skeletal structures; these may, nevertheless, all be derived without violence from a common stem-form, or simple spherical lattice-shell, _Cenosphæra_. The main stem of the order, the extensive suborder #Sphæroidea# (Pls. 5-30), is derived immediately from _Cenosphæra_ (p. 61, Pl. 12); three diverging branches of it being represented by the other three suborders, the #Prunoidea# (Pls. 16, 17, 39, 40) being developed by elongation, and the #Discoidea# (Pls. 31-48) by shortening of the vertical main axis, whilst the #Larcoidea# (Pls. 9, 10, 49, 50) have originated by the modification of the spherical lattice-shell into a lentelliptical or triaxial ellipsoidal one. Although the monophyletic derivation of all #Sphærellaria# from _Cenosphæra_ is exceedingly probable, the possibility of a polyphyletic origin for the group is by no means excluded. For even in the skeletonless primitive genus of all the SPUMELLARIA, _Actissa_ (as well as in the social _Collozoum_), there are found, in addition to the usual spherical types, other species (or subgenera, p. 12) whose central capsule is not spherical but a modification of the sphere; in _Actiprunum_ ellipsoidal; in _Actidiscus_ lenticular; in _Actilarcus_ lentelliptical; if such modified forms of _Actissa_ were to develop their lattice-shells independently, then their form would correspond to that of the central capsule; and such simple ellipsoidal, discoidal, and lentelliptical lattice-shells might have been the primitive forms of the #Prunoidea#, #Discoidea# and #Larcoidea#.