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



reo;ular, and a further distinction from the SpuMELLAPaA consists in the fact that the 

 primary skeletal elements, from which the lattice is secondarily developed, exercise a 

 predominant influence upon their form. These primary elements in the majority of the 

 Nassellaria are to be seen in two morphologically most important structures : — first, the 

 •primary sagittal ring, which embraces the central capsule in the median plane (§ 124); 

 and secondly, the basal tripod (§ 125), whose three diverging rays proceed from the base 

 of the central capsule, whilst commonly a fourth vertical ray supjDorts the dorsal side of 

 latter (compare Pis. 81-91, p. 892). In the majority of the Nassellaria these two 

 primary elements appear in combination, whilst in others only one of them is recognisable. 

 In addition there occur numerous monaxon lattice-shells in which neither of these 

 elements can be recognised, but a simple ovoid lattice-shell (cephaHs) alone forms the 

 whole skeleton or its primary part (PL 51, fig. 13: PI. 98, fig. 13). The great 

 difficulty in the morphological iuterj^retation and phylogenetic derivation of the jSTassel- 

 LARLA.N' skeleton lies in the fact that each of these three elements — the primary sagittal 

 ring, the basal tripod, and the latticed cephalis — may form the whole skeleton by itself or 

 be combined with one or both of the others (p. 893). Even nearly related or at all 

 events very similar forms may diflfer very greatly in this respect. With regard to the 

 manifold forms of their dictyosis it follows that it is partly dependent upon one of the 

 two first elements, partly independent. In the Plectellaria (or those Nassellaria 

 Avhich do not possess a complete lattice-shell) the lattice-work is usually irregular and 

 arises by union of the ramifications, which proceed either from the primary sagittal ring 

 (Pis. 81, 82, 92-94) or from the basal tripod (PL 91). In the Cyrtellaria (or 

 Nassellaria with a complete lattice-shell, Pis. 51-80), on the other hand, the lattice- 

 work is sometimes regular, sometimes irregular, being often very different in the diff"erent 

 joints of a segmented shell (PL 72); a great part of it arises independently of the two 

 chief morphological elements, and develops according to laws similar to those which 

 regulate the dictyosis of the Spumellaria. 



134. Dictyosis of the Phceodaria. — The lattice-structures of the Ph^eodaria, Avhich 

 consist of a silicate of carbon (§ 102), are on the whole not developed in such variety as 

 those of the other Radiolaria, but exhibit several essentially difi"erent t}^es of structure, 

 not reducible to a common primitive type of lattice-work. In one j)ortion of this legion 

 there occurs an ordinary simple lattice-work (as in Spumellaria and Nassellaria), with 

 solid trabecular ; of these the Castanellida (PL 113) and Concharida (Pis. 123-125) 

 have usually regular or subregular, circular meshes, sometimes hexagonally framed ; the 

 Orosphferida (Pis. 106, 107) large irregular polygonal meshes with thick trabeculse, 

 the Sagosphferida (PL 108) large triangular meshes with thin filiform trabeculse. The 

 Challengerida (PL 99) are characterised by a very delicate regular lattice-work, with 

 minute hexagonal pores, like a Diatomaceous frustule. The Medusettida (Pis. 1 18-120) 



