146 PROTOZOA. 



in some cases absent. No dark pigment-body ("phaeodium") in the 

 extra-capsular sarcode. 



This division includes forms such as Thalassicolla and Collozoum, in 

 which the skeleton is wanting, with others having a skeleton of siliceous 

 spicules, and with a large number of the forms usually spoken of as 

 " Polycystiiia " in which the skeleton has the form of a latticed siliceous 

 shell. 



2. Acantharia. — Capsular membrane perforated by numerous fine 

 pores. Fundamental form originally spherical. Skeleton composed of 

 " acanthin." No dark pigment-body in the extra-capsular sarcode. This 

 group includes forms such as Acanthometra and its allies. 



3. Nassellaria. — Capsular membrane perforated by a porous area, or 

 by one single large opening divided into numerous very fine pores. 

 Fundamental form originally egg - shaped. Skeleton siliceous. No dark 

 pigment-body ("phaeodium") in the extra-capsular sarcode. This group 

 comprises all those " Poiycystina" which are not included under the 

 head of Spumellaria. 



4. PHiEODARiA. — Capsular membrane double, perforated by one 

 simple main opening prolonged into a tube, commonly with one or two 

 small accessory openings. Fundamental form originally egg-shaped. 

 A dark pigment-body (" phaeodium ") is constantly present in the extra- 

 capsular sarcode. The skeleton is siliceous, being usually composed of 

 a compound of silica with some organic substance, but in other instances 

 (Dictyocha) consisting of pure silica. 



As regards their distribution in space, the Radiolarians are exclu- 

 sively marine, and are found in all seas and at all depths. They are 

 commonly floating organisms, and are often present in enormous 

 numbers, the greatest variety of specific types, however, being found 

 in the warm seas of the tropics. Many Radiolarians are " pelagic," 

 and inhabit the surface-waters of all oceans ; others are " abyssal," 

 and are confined to great depths in the sea ; while others, again, are 

 " zonarial," and are restricted to particular bathymetrical horizons 

 between the surface and the bottom. Over large areas of the deep 

 sea, principally at depths of from two thousand to over four thousand 

 fathoms, the bottom is found to be covered with extensive deposits 

 of " Radiolarian ooze." The deposit so called is a siliceous mud, 

 with little calcareous matter, which is composed more or less largely 

 of the siliceous tests of various Radiolarians. The skeletons of 

 Radiolarians are, however, also present, in smaller or greater num- 

 bers, in many of the marine deposits which are formed at compara- 

 tively limited depths. 



As regards their distribution in time, the Radiolarians are abun- 

 dantly represented by fossil forms, and are now known to have a 

 high antiquity. Owing, however, to the vast number of forms in- 

 cluded in the Radiolaria, and the great complexity of their classifica- 

 tion, the study of the fossil types can hardly be undertaken except 

 by a specialist. For these reasons, nothing further will be here 

 attempted than merely to give a brief outline of the more important 

 facts relating to the past history of the Radiolaria. The student 



