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



central capsule, are very different in size and dictyosis from all the others which lie outside, and 

 are separated by wider interspaces (compare Pis. 17, 24, 29-32, 40, &c.). In these cases it. appears 

 better to regard the two inner as inner and outer medullary shells, and all the others as cortical 

 shells. The character of the dictyosis in the intracapsular and extracapsular shells is often so 

 different that I have made it the basis of separation of Thecosphcera and Rliodosphcera among the 

 Liosphserida (p. 60), of Elatommatida and Diplosphaerida among the Astrosphrerida (p. 208), &c. 



B. E. Hertwig (1879, L. N. 33, pp. 40, 123) separates the true (simultaneously formed) "cortical 

 shells" (e.g., of Actinomma, Cromyomma) from the arachnoid "siliceous net works" (e.g., of Diplosphcera 

 and Arachnosphcera) which are formed by the successive union of tangential apophyses of the radial 

 spines. Whether this principle is right in theory or not, it cannot be carried out practically. 

 Compare also PI. 25, fig. 4. 



130. Dictyosis or Lattice Formation of the Skeleton. In the great majority of 

 Radiolaria the dictyosis or formation of lattice-work, and especially the formation of a 

 variously-shaped " lattice-shell," plays such an important part that the whole class has 

 long been popularly known in Germany by the name " lattice animalcules " (" Gitter- 

 thierchen " or " Gitterlinge ") (Protista dictyota). The old name Polycystina also (1838), 

 although referring only to the SPUMELLARIA and NASSELLABIA, is derived from the 

 lattice-work of the siliceous skeleton. The extremely various forms in which this is 

 manifested furnish the means of distinguishing species. The specific conformation of the 

 skeletal lattice-work is usually caused by the special disposition of the sarcodictyum ( 94), 

 whose exoplasmatic threads become silicified or (in the ACANTHARIA) converted into bars 

 of acanthin. In many cases, however, the form of the lattice is mainly dependent upon 

 the situation and form of the radial spines or of special processes from them. With 

 respect to their origin, two varieties of lattice may be distinguished simultaneous and 

 successive. Simultaneous dictyosis occurs especially in the simple lattice-shells of the 

 Sphserellaria and PH^EODARIA, where, at a given moment (" dictyotic moment") 

 the whole lattice of the shell is excreted on the surface of the calymma. Successive 

 dictyosis, on the other hand, is found more particularly in the lattice-shells of the 

 ACANTHARIA (and in the concentric cortical shells of many Sp h se r e 1 1 a r i a), which 

 develop from separate lattice-plates formed by the apophyses of the radial spines, 

 and hence not at the same moment. The lattice-shells of the Cyrtellaria, which 

 gradually grow out from a sagittal ring or a basal tripod, arise by successive dictyosis. 



131. Dictyosis of the Spumellaria. Siliceous lattice-structures are wanting in the 

 first section of the SPUMELLARIA, the Collodaria, but in the second section, S p h 33 r- 

 e 1 1 a r i a, they are developed in extraordinary variety of details. In spite of this extreme 

 richness in different forms, the lattice-shells of the SPUMELLARIA may all be derived from 

 one and the same primitive ground-form, a simple lattice-sphere with regular hexagonal 

 meshes (Phormosphcera, p. 61, PI. 12, figs. 9-11 ; Heliosplmra, PI. 28, figs. 1-3, &c.). 



