The genus _Actissa_ is the most simple and typical form of all Radiolaria, and may be regarded as the common ancestral form of the whole class. The spherical body is composed of a simple spherical capsule and a concentric spherical calymma or jelly-envelope. Neither the former nor the latter contains alveoli. The central capsule possesses a strong membrane perforated by small pores, and contains in the intracapsular sarcode numerous small pellucid vacuoles, and in its middle a simple, concentric, spherical nucleus (often with some nucleoli); sometimes also one or more oil-globules. The extracapsularium forms a soft, voluminous, structureless calymma or enveloping jelly-sphere, perforated by the numberless, fine pseudopodia, which radiate outwards from the matrix or the thin granulated sarcode-layer, surrounding the capsule. Often (but not constantly) xanthellæ or yellow cells are scattered in it. _Actissa_ differs from the following skeletonless genera in the absence of all alveoli; it has neither intracapsular alveoli (like _Thalassolampe_) nor extracapsular alveoli (like _Thalassicolla_). The first observed species of this genus is that which I found in 1866 at the Canary Islands, _Actissa prototypus_; the second is that which Hertwig accurately described in 1879, from the Mediterranean (Messina), _Actissa primordialis_; the third I observed in 1881 at Ceylon, frequent and sporiparous, _Actissa princeps_. A fourth species (_Actissa radiata_) exhibited a distinct radial segmentation of the capsule-contents. These four species are quite spherical. Six other species, occurring in different preparations from the Challenger, are distinguished by modifications of the spherical capsule-form and may represent three different subgenera (or, perhaps better, genera?)--_Actiprunum_ ellipsoidal, _Actidiscus_ lenticular, _Actilarcus_ lentelliptical; perhaps these are the ancestral forms of the three sections: #Prunoidea#, #Discoidea#, #Larcoidea#. {13}Subgenus 1. _Procyttarium_, Haeckel, 1879. _Procyttarium_, Haeckel, Natürl. Schöpfungsgeschichte, ed. vii. p. 705. _Definition._--Central capsule spherical. 1. _Actissa princeps_, n. sp. (Pl. 1, fig. 1).