EEPOET ON THE RADIOLARIA. CXXIU 



ris. 52 54, &c.). As regards the numhcr of radial apojihyses, three sections of Cyrtoidea may- 

 be distinguished ; the Pilocyrtida with three, the Astrocyrtida with numerous apophyses, and the 

 Corocyrtida with none (p. 1129). The last two may in general be regarded as two divergent 

 branches from the first, for the eradiate Corocyrtida have probably been formed from the triradial 

 Pilocyrtida by entire loss of the radial apophyses, whilst on the other hand the multiradiate 

 Astrocyrtida have arisen from them by additions to the three primary apophyses (interpolation of 

 interradial between the perradial ones). As regards the constitutimi of the shell-aperture, the 

 Cyrtoidea may be divided into Cyrtaperta and Cyrtoclausa (p. 1129); in general the 

 Cyrtoclausa (with latticed shell-aperture) have arisen from the Cyrtaperta (with simple open 

 mouth) ; in many Monocyrtida the converse may be supposed, the simple basal mouth having been 

 formed by degeneration of a basal lattice. 



192. PhylogemjofthePhceodaria. — The legion PH.i:oDARiA or Cannopylea is so 

 clearly marked off from other Radiolaria by the double membrane of the central capsule 

 and the astropyle at its oral pole, as well as by the extracapsular phaeodium, that it 

 must be regarded phylogenetically as an independent stem (§ 9). This stem is only 

 connected at its root by Phceodina with the stem-form of the Spumellaria, Actissa. 

 The stem itself is mouophyletic, inasmuch as its members may be derived without 

 violence from the skeletonless Phseodinida [Phceodina, PhcBOColla). On the other hand, 

 the formation of the skeleton of the Ph^eodaria is undoubtedly polyphyletic, different 

 Phseodinida having independently commenced the formation of a skeleton, and having 

 carried it o^^t in very different ways. 



193. Origin of the Phceodaria. — The Phseodinida (p. 1544, PL 101), which may 

 naturally be regarded as the common stem -group of the Ph^odaria, have their nearest 

 relations among other Eadiolaria in the Thalassicollida (p. 10); and since this family is 

 to be regarded as the primitive group of all Radiolaria, they may be directly derived 

 from them phylogenetically. The essential modifications by which the primitive 

 Phseodinida have arisen from the more archaic Thalassicollida are of three kinds; (1) 

 the doubling of the membrane of the central capsule ; (2) the reduction of the 

 numerous fine pores in the membrane and the formation of an osculum, and of an 

 astropyle closing it, at the oral pole of the main axis ; (3) the production of an extra- 

 capsular phseodium. This last may, perhaps, be regarded as a unilateral hypertrophy 

 of the voluminous pigment masses which are deposited in the sarcomatrix of certain 

 Thalassicollida. Of the two genera of Phseodinida hitherto known, probably Phceodina 

 (PI. 101, fig. 2) approaches the original stem of the Phceodaria more nearly than 

 Phceocolla (PI. 101, fig. 1), for the latter exhibits only the large main opening of the 

 central capsule (astropyle), whilst the former possesses also a pair of accessory openings 

 (parapylse). The hypothetical stem-form {PhcBometra) presumably had a larger number 

 of small parapylas (like many Cii-coporida and Tuscarorida), and the astropyle was 

 probably but little differentiated from them. 



