REPORT ON THE RADIOLARIA. Ixxiii 



small group A c t i u e 1 i d a) radial spines are centrifugally developed. The three other 

 legions, on the contrary, possess on the whole a perigenous skeleton, which originally 

 develops outside the central capsule and never in its middle. In the Nassellaria and 

 Ph^odaria the skeleton retains this extracapsular position, as also in the B e 1 o i d e a 

 and part of the Sphger ell aria among the Spumellaria ; in the great majority of 

 the latter, however, the primary perigenous skeleton is subsequently enveloped by the 

 growing central capsule, so that it lies partially within it (§ 109). 



108. Polyphyletic Origin of the Skeleton. — The skeleton of the Eadiolaria has un- 

 doubtedly originated polyphyletically, for it is impossible to derive its manifold varieties 

 from a single ground-form, or to regard them as modifications of one type. It is much 

 more probable that the different skeletonless Eadiolaria have entered upon different ways 

 of skeleton formation quite independently of each other. At the outset it is quite clear 

 that the skeletons of the four legions have originated independently of each other. 

 Further, it is certain that within the leoion of the Spumellaria the Beloid skeletons of 

 the Collodaria are not connected with the Sphseroid skeletons of the Sphserellaria 

 and the forms derived from them (see § 109). In the same way the skeletons of the 

 Ph^odakia are polyphyletic ; probably in this legion the Beloid, Spheeroid, Cyrtoid, 

 and Conchoid skeletons have been developed cpite independently (see § 112). In the 

 Nassellaria, on the other hand, it is possible that all the skeletal forms are to be derived 

 monophyletically from a single simple primitive form (either the sagittal ring or basal 

 tripod?) (see § 111). Still more probable is it that the Acantharia have arisen mono- 

 phyletically, for all the forms of their acanthin skeleton may be derived without violence 

 from Actinelius (see § 110). 



109. The Skeleton of the Spumellaria. — The skeletons of the Spumellaria or Peri- 

 pylea consist of silica, and are very different and of independent origin in the two orders 

 of this legion. The first order, Collodaria, have either no skeleton whatever (C o 1- 

 loidea, p. 10, Pis. 1, 3), or their skeleton is Beloid, a loose extracapsular envelope of 

 spicules, consisting of numerous unconnected portions ; the separate parts are usually 

 disposed tangentially, either as simple or compound siliceous spicules (B e 1 o i d e a, p. 28, 

 Pis. 2, 4). The second order of Spumellaria, on the other hand (Sphserellaria, 

 p. 49), develops a siliceous lattice-shell, which consists of a single piece, and is remarkable 

 for the extraordinary variety of its forms (pp. 50-715, Pis. 5-50). To this order belong 

 not less than three hundred genera and seventeen hundred species of the Challenger 

 Eadiolaria (that is, about two-fifths of all the genera and species). In spite of this 

 extreme richness in different forms, this large group must be regarded as monophyletic, 

 since all its forms may be quite naturally derived from a common stem-form, a simple 

 lattice- sphere {Cenosphcera, p. 61, PI. 2). The twenty-eight families of S p h ae r e 1 1 a r i a 

 may be distributed in four suborders, among which the Sphasroidea constitute the 



(ZOOL. CHALL. EXP.— PART XL. 1886.) Rl' k 



