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



further demonstration of this was furnished by Biitschli in the general account of the 

 organisation of the Radiolaria which he published in 1882 in Bronn's Klassen und 

 Ordnungen des Thierreichs (L. N. 41). 



In our knowledge of the developmental history of these Protista the last two de- 

 cades have witnessed less progress than in their comparative anatomy. The most 

 important advance in this direction has been the proof that in all the main groups of 

 the class the contents of the central capsule are used in the formation of swarm-spores. 

 The movements of these zoospores in the central capsule had indeed been observed by 

 several previous authors in the case of the SPUMELLAKIA and ACANTHAEIA (L. N. 10, 

 13, 16; compare also 142, Note A). The origin of the flagellate spores from the 

 contents of the central capsule and their peculiar constitution were, however, first de- 

 scribed fully by Cienkowski in 1871 (L. N. 22, p. 372). Soon after this, R. Hertwig 

 discovered that in the social Radiolaria (Polycyttaria or Sphserozoea) two different forms 

 of zoospores are formed, one with, the other without crystals, and that the latter are 

 also divided into macrospores and microspores (compare L. N. 26, and 142). Recently 

 this sexual differentiation has been shown by Karl Brandt to exist in all the groups of 

 Sphaerozoea, and its regular interchange with the formation of crystal-spores has been 

 interpreted as a true "alternation of generations" (compare L. N. 52 and also 216). 

 The other forms of development also, especially reproduction by cell-division ( 213) 

 and gemmation ( 214), have been elucidated by the recent investigations of the same 

 author. 



The palaeontology of the Radiolaria has of late made important and interesting 

 advances. Until ten years ago fossil remains of this class were known exclusively from 

 the Tertiary period ; almost the only source of our information was to be found in the 

 researches of Ehrenberg, commenced in 1838, continued in his Mikrogeologie in 1854, 

 and concluded in his last work (L. N. 25) published in 1875 (compare L. N. 16, pp. 

 39, 191193). In the year 1876 a number of Mesozoic Radiolaria from the chalk 

 were described by Zittel (L. N. 28), and afterwards others from the Jura by Dunikowski 

 (L. N. 44). That fossil Radiolaria occur in Mesozoic formations, especially in the Jura, 

 as well preserved and as abundantly as in the Tertiary rocks of Barbados, was shown in 

 1883 by Rust (L. N. 48). By the examination of numerous thin sections he discovered 

 that in all the main divisions of the Jurassic formation (Lias, Dogger, Malm) there are 

 distributed jaspers, flints, cherts, and other quartzites, which consist largely of the 

 siliceous shells of Polycystina ; the same is true also of many Coprolites found in the 

 Jura. The full account of these and the descriptions and figures of 234 Jurassic species, 

 distributed in 76 genera, are contained in the Beitrage zur Kentniss der fossilen Radio- 

 larien aus Gesteinen des Jura (L. N. 51, 1885). But even in the older rocks, the 

 Trias, the Permian, and Carboniferous systems, and even as far downwards as the Silurian 

 and Cambrian formations, Rust has recently shown the existence of fossil Radiolaria, 



