160. _Origin of the Spumellaria._--The genus _Actissa_ (p. 12, Pl. 1, fig. 1) presents the Radiolarian type in its simplest and most primitive form--a spherical central capsule, which encloses in its middle a spherical nucleus, and which is surrounded by a spherical calymma. The whole unicellular body consists, therefore, of three concentric spheres, {cvi}and possesses neither skeleton nor alveoles, nor other differentiated parts. The innumerable fine pseudopodia, which issue from the central capsule through the evenly distributed pores in its membrane, radiate in all directions through the calymma and pass out over its surface. _Actissa_ can, therefore, be directly derived phylogenetically from the simplest skeletonless Heliozoa (_Actinophrys_, _Heterophrys_, _Actinosphærium_, _Sphærastrum_). The only essential difference between the two consists in the development of the _central capsule_, which in _Actissa_ separates as a distinct membrane the endoplasm from the exoplasm. This differentiation which we regard is the most important distinguishing character of the Radiolaria, has been transmitted by inheritance, along with the formation of flagellate spores in the central capsule, from _Actissa_, the primitive parent to all the other Radiolaria. {cvii}161. _Hypothetical Genealogical Tree of the Spumellaria_:-- LARCOIDEA DISCOIDEA ~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~ Streblonida PHACODISCARIA | Coccodiscida | THOLONIDA | | | | PRUNOIDEA | | | ~~~~~~~~~~~~~~~~~~ Soreumida| | | | | | | Zygartida | | | | | | | | | | | |Zonarida| | | |Lithelida| | SPHÆROIDEA |CYCLODISCARIA | | | | |~~~~~~~~~~~~~ | Spongodiscida | | | | |Stylosphærida |Pylodiscida| | Phorticida| | | | | | | | | | | | | | | | Panartida | | | | | | | | | Artiscida| +----+---+ | | | | | | | | | | | | | | | | | Phacodiscida| | |Spongodruppida | | | | | | | | | | | | Staurosphærida | +-----+ | | | | Pylonida | | | | | | | | | | |