170. _Origin of the Acantharia._--The genus _Actinelius_ (p. 730, Pl. 129, fig. 1), which may naturally be regarded as the common primitive form of all ACANTHARIA, possesses a spherical central capsule, which in consequence of the early division of the nucleus (§ 63), encloses numerous small nuclei; from its centre arise many simple radial spines of equal size, which penetrate the central capsule. A large number of radial pseudopodia issue between the spines from the sarcomatrix which surrounds the capsule. _Actinelius_ may have been directly derived from _Actissa_, the common stem-form of all Radiolaria, by the division of the pseudopodia into two groups, myxopodia, which remained soft, and axopodia, which became firm (§ 95A). As the latter became changed into strong acanthin rods, and touched each other in the centre, they forced the nucleus from its originally central position and brought about its early division. _Actinelius_ is also of all Radiolaria the form which, next to _Actissa_, most nearly approaches the Heliozoa. If the stiff axial threads of _Actinosphærium_ be conceived of as partially converted into acanthin spines, and its nucleated medullary substance as separated from the alveolar cortical layer by a membrane (central capsule), then _Actinelius_ would be produced. {cxiii}171. _Hypothetical Genealogical Tree of the Acantharia_:-- Diploconida | | Phractopeltida Hexalaspida Cenocapsida | | | | Phatnaspida | Lychnaspida | | | | | | | | | | Porocapsida | | Coleaspida | | | | | | | | Ceriaspida +-----+-----+ | | | | | | | +--+--+ | | | | Belonaspida | | | | | | Phractaspida | Stauraspida | | | | | | | | Astrocapsida | | | Sphærocapsida +-------+----------+ ------+------ | | | | Diporaspida Tessaraspida | (Dorataspida dipora) (Dorataspida tetrapora) | | | | | | | +--------+--------+ | | | | | |