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



Diporaspida in the same way as the Dyosphserida from the Monosph^rida among the 

 S p h fe r o i d e a ; in that case the smaller inner lattice-sphere (medullary shell) would be 

 the primary, and the larger outer sjihere (cortical shell) the secondary ; this latter shows forty 

 primary aspinal pores like those of the Diporaspida. The possibility is not excluded, 

 however, that the small inner lattice-sphere of the Phractopeltida is a secondary product. 

 The three remaining families, which must be regarded as descendants of the Diporaspida, 

 form together a single phylogenetic series, and are separated from the primitive group 

 mainly by the fact that the original spherical form of the lattice-shell has been modified 

 into one distinguished by an elongated equatorial axis (the hydrotomical axis); hence 

 the Prun op hracta (pp. 794-859). The ellipsoidal Belouaspida have arisen directly 

 l)y h}q3ertrophy of the two opposite ecpiatorial spines of this hydrotomical axis (p. 859, 

 PI. 136, figs. 6-9 ; PL 139, figs. 8, 9 ; perhaps they have also arisen directly from the 

 Amphilonchida). In the lentelliptical Hexalaspida (PI. 139) all six spines which lie in 

 the hydrotomical meridian plane (two equatorial and four polar) are very strongly 

 developed, the remaining fourteen being rudimentary. Finally, in the Diploconida the 

 two conical sheaths of the two opposite hydrotomical equatorial spines are so predominant 

 that they take the chief part in the formation of the hour-glass-shaped shell. 



178. Plujlogeny of the Nassellaria.— The legion Nassellaria orMoNOPYLEA is so clearly 

 characterised by the peculiar porochora, which closes the osculum at the oral pole of the 

 monaxon central capsule, and by the podoconus connected with it, that there can be no 

 douljt that phylogenetically it represents an independent stem (§ 8). This stem is only 

 connected at its base by means of Cystidium and Nassella with Actissa and Thalassi- 

 colla, the stem-forms of the Spumellaeia. This stem is monophyletic, inasmuch as 

 all its members may be derived without violence from the skeletonless NasseUida 

 (Nassella, Cystidium, p. 896, PI. 91, fig. 1). 



179. Origin of the Nassellaria. — The NasseUida (p. 896), which may naturally be 

 considered as the common stem-group of the Nassellaria, are most nearly related 

 amono- other Radiolaria to the Thalassicollida, and in both these skeletonless families the 

 simplest forms, Cystidium .and Actissa correspond ; on the other hand, those which have 

 arisen from them by the formation of alveoles in the calymma (N'assella and Thalas- 

 sicolla) also correspond. The origin of the simplest NasseUida from these primitive 

 Thalassicollida may be explained by supposing that the numerous (formerly evenly 

 distributed) pores of the capsule membrane became obliterated in the upper (apical) half 

 of the central capsule, whilst in the lower (basal) half they became correspondingly more 

 strongly developed ; hence the porochora was formed at the oral 23ole of the vertical main 

 axis, and a differentiation of the endoplasm proceeding from this gave rise to the 

 characteristic podoconus. Both these organs still at present exhibit very various 

 degrees of progressive development. 



