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



may be uuited as Cyr tell aria. The former correspond to the Acanthodesmida, 

 the latter to the .Cyrtida in my Monograj^h. 



The character common to all Monopylea or Nassellaria, which separates them from 

 all other Radiolaria, was first recognised by Richard Hertwig in 1879, and consists in 

 the singular structure of the monaxonian central capsule, bearing on the basal pole a 

 peculiar porous area or operculum, the "Porenfeld;" we call it shortly the " porochora." 

 It represents a circular or elliptical porous plate on the basal pole of the vertical main 

 axis of the central capsule, and bears a peculiar " podoconus " or " Pseudopodien-Kegel," 

 a conical body of singular structure, protruding inside the membrane into the capsule. 

 The pseudopodia arising from this peculiar " podoconus " proceed from the capsule 

 piercing the " porochora," whilst the other parts of the capsule are not perforated. 



The MoNOPYLEA are therefore " Merotrypasta," like the following fourth legion, the 

 Ph.^odaria or Cannopylea. But in these latter we find on the basal pole of the 

 monaxonian capsule only one single large main opening, prolonged into a peculiar tube, 

 and there is no trace of the typical " podoconus," characteristic of all Nassellaeia. 

 The latter agree, however, with the former in the possession of a basal opening, serving 

 for the emission of the pseudopodia, and in the monaxonian fundamental form, arising 

 from this structure. Therefore these two legions of " Merotrypasta " exhibit a wider 

 divergence from the Acantharia and Spumellaeia, the two legions of " Holotrypasta," 

 in which the central capsule is everywhere perforated by innumerable small pores (com- 

 pare above, pp. 5, 6, 716). 



Tlie Skeleton in all Nassellaria consists either of pure silica, or of a peculiar 

 silicate ; never of acanthin (as in all Acantharia). The siliceous bars and beams 

 constituting it are invariably solid (as also in the Spumellaria) ; never hollow 

 (as in the Ph^odaeia). In the small family of Nassellida alone (with the two 

 genera Cystidium and Nassella) the skeleton is entirely absent. In all other genera 

 of Nassellaria the siliceous skeleton is more or less developed ; imperfect, or quite 

 rudimentary in the Plectellaria (Plectoidea and S t e p h o i d e a), but 

 perfect and forming a regular lattice-shell in the C y r t e 1 1 a r i a (S p y r o i d e a, 

 Botryodea, and C y r t o i d e a). The different forms of this skeleton exhibit an 

 extraordinary variety, but may be reduced to a few very simple fundamental forms, or 

 even to a single, most simple original form. The comparative morphology is more 

 interesting, but also more difficult to understand than in any other Radiolaria. 



The geometrical fundamental form of the skeleton is in all Nassellaria monaxonial, 

 the vertical main axis of the body, which is already indicated by the axis of the central 

 capsule with two difi'erent poles, being also expressed constantly in the form of the 

 skeleton. The lower or basal pole of the latter always exhibits a difierent shape from 

 that of the upper or apical pole. This difi"erence is so striking in nearlj^ all Monopylea, 

 that the two poles may be determined on the first view. 



