EEPORT ON THE RADIOLARIA. CV 



D i s c i d e a and S p li re r o i d e a among the Spdmellaria. From these palseontoLogical facts it 

 is obvious that our present very incomplete acquaintance with the fossil Eadiolaria is quite insuffi- 

 cient to warrant us in drawing any conclusions from it regarding the phylogenetic development or 

 palKontological succession of the individual groups. 



158. Origin of the Four Legions. — The agreement of all Radiolaria in those constant 

 and essential characters of the unicellular body, which distinguish them from all other 

 Protista (especially the differentiation of the malacoma into a central capsule and 

 extracapsulum), justifies the conclusion that all members of this class have been developed 

 from a common uudifierentiated stem-form. Only the simplest form of the Spumellaria, 

 a skeletonless spherical cell with concentric spherical nucleus and calymma, can be 

 regarded as such. The simplest form of the ThalassicoUida which is now extant (Actissa, 

 Procyttariurti, p. 12), corresponds so exactly to the morphological idea of that hypothe- 

 tical stem-form that it may unhesitatingly be regarded in a natural system as the common 

 point of origin of the whole class. On the other hand, Actissa is so closely related to 

 the simple Heliozoa (Actinophrys, Actinosphcerium, Heterophrys, SphcBrastrum, &c.) that 

 its origin from this group of Rhizopoda is exceedingly probable. The three legions 

 AcANTHARiA, Nassellaria, and Ph^odaeia are to be regarded as three main diverging 

 branches of the genealogical tree, which have been developed in different directions and 

 are only connected by their simplest stem-forms (Actinelius, Nassella, Plueodina) with 

 the stem-form of the Spumellaria, the primordial Actissa. 



159. Phytogeny of the Spumellaria. — The legion Spumellaria or Peripylea is to be 

 regarded as the common stem-group of the Radiolaria, and its simplest form, Actissa, as 

 the primitive genus or radical form of the whole class ; for it possesses in the simplest and 

 most undifferentiated form all those characters by which the Radiolaria are distinguished 

 from other Protista ; aU the other genera of the class may be derived from it by succes- 

 sive modifications. Considered as a legion the whole group Spumellaria is undoubtedly 

 monophyletic, for all its members possess those essential characters by which it is dis- 

 tinctively marked off from the other three legions, more especially a simjale capsule- 

 membrane, which is everywhere evenly perforated by innumerable small pores ; the nucleus 

 lies originally in the centre of the spherical central capsule. Furthermore, all Spumellaria 

 lack those positive characters which distinguish the three remaining legions — the centro- 

 genous acanthin skeleton of the Acantharia, the basal porochora and the monaxon podo- 

 conus of the Nassellaria, the astropyle and phseodium of the Ph^odaria. 



IGO. Origin of the Spumellaria. — The genus Actissa (p. 12, PL 1, fig. l) 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, 



(ZOOL. CHALL. EXP. — PART XL. — 1886.) Rf 



