The division of the Radiolaria into two subclasses and four legions (or principal orders), I sought to establish in 1883 in a communication on the Orders of the Radiolaria (Sitzb. Jena Gesellsch. Med. u. Naturwiss., February 16, 1883). As a believer in the theory of descent, I regard all the systematic arrangements of specialists as artificial, and all their divisions as subjective abstractions, and hence I shall be guided in the establishment of such groups as subclasses, legions, orders, &c., by purely practical considerations, especially by the desire to give as ready a survey as possible of the complex multitude of forms (compare §§ 154 to 156). 3. _Porulosa or Holotrypasta._--The subclass Porulosa or Holotrypasta includes the two legions, PERIPYLEA or SPUMELLARIA, and ACTIPYLEA or ACANTHARIA, which agree in the following constant and important characters:--(1) The _Central Capsule_ is primitively a sphere, and retains this homaxon form in the majority of the species. (2) The _Membrane_ of the central capsule is everywhere perforated by very numerous minute pores, but possesses no larger principal aperture (osculum). (3) The _Pseudopodia_ radiate in all directions and in great numbers from the central capsule, passing through its pores. (4) The _Equilibrium_ of the floating unicellular body is in most Porulosa pantostatic (indifferent) or polystatic (plural-stable), since a vertical axis is either absent, or, if present, has its two poles similarly constituted. (5) The _Ground-forms_ of the skeleton are therefore almost always either spherotypic or isopolar-monaxon, very rarely zygotypic. The two legions of the Porulosa are distinguished mainly by the skeleton of the SPUMELLARIA (or PERIPYLEA) being siliceous, never centrogenous, nor composed of acanthin, whilst in the ACANTHARIA (or ACTIPYLEA) it is always centrogenous and made up of acanthin; hence in the former the nucleus is always central, in the latter always excentric.