ii THE VOYAGE OF H.M.S. CHALLENGER. 



1884, contains 20 orders, 85 families, 739 genera, and 4318 species. The consideration 

 that but a small proportion of the ocean has yet been investigated renders it likely, 

 however, that even this large number does not include the half of the recent species. 

 The great progress which our knowledge of the organisation of the Eadiolaria has 

 made, by means of comparative study, renders it possible to arrange this enormous 

 mass of forms in four main divisions or legions, and these are again related in pairs, so 

 that two divisions of the highest rank or subclasses are constituted, the Porulosa 

 (or Holotrypasta) and Osculosa (or Merotryjyasta). 



The division of the Eadiolaria into two subclasses and four legions (or principal orders), 

 I sought to establish in 1883 in a communication on the Orders of the Eadiolaria (Sitzb. Jena 

 GeseUsch. 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 aU 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 Acanthaeia, which agree in 

 the following constant and important characters : — (l) The Central Capsule is primi- 

 tively a sphere, and retains this homaxon form in the majority of the species. (2) The 

 Memhrane 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 pre- 

 sent, has its two poles similarly constituted. (5) The Ground-forms of the skeleton are 

 therefore almost always either spherotj^pic 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 acan- 

 thin ; hence in the former the nucleus is always central, in the latter always excentric. 



4. Osculosa or Mey^otrypasta. — The subclass Osculosa or Merotrypasta includes 

 the two legions Monopylea or Nassellaria, and Cannopylea or Ph^odaria, which 

 agree in the following constant and important characters : — (1) The Central Capside is 

 originally monaxon (ovoid or spheroidal) and retains this ground-form in most of 

 the species. (2) The Membrane of the central capsule possesses a single large principal 

 aperture (osculum) at the basal pole of the vertical main axis. (3) The Pseudopodia 

 radiate from a stream of sarcode which passes out from the central capsule only on one 

 side, namely, through the principal aperture. (4) The Equilibrium of the floating body is 



