REPORT ON THE RADIOLARIA. Ixxxvii 



The siliceous bars which bound these regular and subregular meshes are at first exceed- 

 ingly thin and filiform ; afterwards they become thicker or spread out laterally, whence 

 the meshes often become round with a hexagonal frame (PL 12, fig. 5 ; PL 28, fig. 1). 

 If the latter vanish, a lattice-shell with simple circular meshes is formed. Very 

 commonly the regular form of the meshes or pores becomes more or less irregular, 

 polygonal, or roundish. Hence, in general, four different principal forms of dictyosis 

 may be distinguished among the SPUMELLARIA ; viz. (l) regular or subregular hexagonal 

 meshes ; (2) regular or subregular circular meshes ; (3) irregular polygonal meshes ; (4) 

 irregular roundish meshes. The three latter forms are to be regarded as secondary, 

 derived from the primary first form. In those SPUMELLARIA which possess several con- 

 centric lattice-shells enclosed one within another, either these have all the same form of 

 dictyosis, or the lattice-work of the innermost primary shell is different from that of the 

 outer secondary shells (Pis. 19, 20) ; sometimes these latter also differ more or less 

 among themselves ( 129). 



132. Dictyosis of the Acantharia. The lattice-structures of the ACANTHARIA differ 

 essentially from those of other Radiolaria in several particulars. Firstly, they consist not 

 of silica but of acanthin ( 102) ; secondly, they are always secondary formations, usually 

 developed from transverse processes of the primary centrogenous radial spines ; thirdly, 

 their formation is not simultaneous (at the same time over the whole shell), but successive 

 (proceeding from the individual radial spines tangentially towards the middle of the 

 intervals) ; fourthly, the configuration of the network is due to the relative position of 

 the spines and the mode of union of their transverse apophyses. Since these are at right 

 angles to the spines, and since the branches of the apophyses are at right angles to them, 

 the original ground-form of their dictyosis is a lattice-work with quadrangular meshes; 

 these are often quite regular and square (PL 130, figs. 5, 6 ; PL 136, figs. 2, 9, &c.); 

 more commonly they are rectangular or irregularly quadrangular (PL 1 3 1 , fig. 10; PL 133, 

 figs. 2, 3, &c.). In the majority of the ACANTHARIA the quadrangular form of the meshes 

 passes over into an irregularly polygonal or roundish one (Pis. 137, 138). Very often the 

 primary meshes of the lattice -shells, which immediately surround the radial spines, are 

 larger and more regular (" aspinal pores "), whilst the numerous secondary meshes between 

 them are smaller and irregular ("coronal pores"; PL 135, figs. 1-4, &c.). 



133. Dictyosis of the Nassellaria. The siliceous lattice-structures of the NASSELLARIA 

 are formed on the whole like those of the SPUMELLARIA, with which they were formerly 

 united under the name " Polycystina." In this group also there may be distinguished 

 as two main forms the regular and the irregular. In the NASSELLARIA the regular lattice- 

 structures generally exhibit hexagonal or circular meshes, whilst the irregular are either 

 polygonal or roundish ; the irregular forms are, however, much more abundant than the 



