RADIOLARIA. 881 



The nuclei of the spores in Thalassicolla nucleata (and probably other 

 species of the genus) are said to be derived from the nucleoli of the single 

 nucleus (Hertwig). 



The simple Radiolaria are mostly of small size, under --$ in. Thalas- 

 solampe maxima (Colloidea) reaches in. circa, but the Phaeodarian family 

 Coelographida contains the giants of the class, several species reaching 

 -|in. and Coelothamnus maximus J^in. The dimensions of the colonies 

 vary much. The globular Myxosphaera coertdea may have a diameter of 

 \ in. ; the elongated colony of Collozoum inerme is nearly i in. long, whilst 

 C. pelagicitim has not rarely a length of 4 in., and has been observed over 

 10 in., with a thickness of % in. Most Radiolaria are phosphorescent. 

 The light centres round the intracapsular oil-globules, and is emitted 

 readily in response to physical or chemical stimuli. With the exception 

 of the Phaeodaria, most Radiolaria contain ' yellow cells ' or symbiotic 

 algae ; see p. 243. They are either extracapsular, or in Acantharia intra- 

 capsular. They may be present or absent in a given species, and when 

 present their numbers are inconstant. The extracapsular cells possess 

 a distinct cell-membrane of cellulose, a nucleus, yellow pigment granules 

 and granules of amylum, i. e. starch, or of an amylum-like substance. 

 They multiply by fission within the cell-membrane, each cell giving origin 

 to four which then escape. If removed from their host artificially, or 

 naturally by its death, the cell-membranes swell up and gelatinise ; the 

 cells themselves become amoeboid and multiply by binary fission. They 

 may escape from their gelatinous coats, and form them anew repeatedly ; 

 but if brought into a plentiful supply of water, they wander out and 

 become biflagellate. They are destroyed however during the sporulation 

 of some colonial forms with assimilation-plasma (note i, p. 878), e.g. 

 Collozoum inerme. The intracapsular yellow cells of the Acantharia 

 occur most commonly in the Acanthometra, rarely in the AcanthopJiracta. 

 They are sometimes found outside the capsule, and appear to be devoid 

 of a cell-membrane. Both forms of cells are supposed to contribute 

 oxygen and starch to their host. Radiolarians appear to be capable of 

 nourishing themselves also in the ordinary way by their pseudopodia, 

 digestion taking place either superficially or sometimes even within the 

 calymna when there is no shell. Living diatoms have been observed 

 within a Collozown and its allies ; Myxosphaera coendea and young Collozoa 

 are infested parasitically by a species of Hyperia (Amphipod). Thalassi- 

 colla sangtiinolenta frequently takes up coccoliths and coccospheres, &c., 



colonial Sfumellaria is divided by Brandt into (i) the spore- stage, (2^/ the young vegetative stage, 

 (3) the young reproductive stage when extracapsular bodies or their analogues are formed, (4) the 

 old vegetative stage, and (5) the old reproductive or fructificative stage when sporulation takes place. 

 It may be noted that Brandt believes the peculiar nucleus of Acanthometrida, described by Hertwig 

 (Dk. Jen. Ges. ii. pp. 148-53), to be an internal bud, which is probably extruded, and grows into an 

 'Acanthometra ; see p. 209 of his monograph cited p. 883. 



