107. _Perigenous and Centrogenous Skeletons._--Much more important than the topographical relation of the skeleton to the central capsule, according to which the Ectolithia and Entolithia are separated from each other (§ 106), is the original development of the skeleton within or without the central capsule, which gives rise to the distinction between perigenous and centrogenous skeletons. _Centrogenous skeletons_ are found only in the ACANTHARIA, which are further distinguished from all other Radiolaria by their skeleton being formed of acanthin; in all ACANTHARIA the formation of the skeleton begins in the middle of the central capsule, from which twenty (the number is inconstant only in the {lxxiii}small group #Actinelida#) radial spines are centrifugally developed. The three other legions, on the contrary, possess on the whole a _perigenous_ skeleton, which _originally_ develops outside the central capsule and never in its middle. In the NASSELLARIA and PHÆODARIA the skeleton retains this extracapsular position, as also in the #Beloidea# and part of the #Sphærellaria# among the SPUMELLARIA; in the great majority of the latter, however, the primary perigenous skeleton is subsequently enveloped by the growing central capsule, so that it lies partially within it (§ 109). 108. _Polyphyletic Origin of the Skeleton._--The skeleton of the Radiolaria has undoubtedly originated polyphyletically, for it is impossible to derive its manifold varieties from a single ground-form, or to regard them as modifications of one type. It is much more probable that the different skeletonless Radiolaria have entered upon different ways of skeleton formation quite independently of each other. At the outset it is quite clear that the skeletons of the _four legions have originated independently of each other_. Further, it is certain that within the legion of the SPUMELLARIA the Beloid skeletons of the #Collodaria# are not connected with the Sphæroid skeletons of the #Sphærellaria# and the forms derived from them (see § 109). In the same way the skeletons of the PHÆODARIA are polyphyletic; probably in this legion the Beloid, Sphæroid, Cyrtoid, and Conchoid skeletons have been developed quite independently (see § 112). In the NASSELLARIA, on the other hand, it is possible that all the skeletal forms are to be derived monophyletically from a single simple primitive form (either the sagittal ring or basal tripod?) (see § 111). Still more probable is it that the ACANTHARIA have arisen monophyletically, for all the forms of their acanthin skeleton may be derived without violence from _Actinelius_ (see § 110).