REPORT ON THE RADIOLARIA. Iv 



their colonies or ccenobia is to a great extent determined by the development, number, size 

 and arrangement of the alveoles in their calymma (compare Pis. 3-8). In these cases 

 there is not unfrequently developed a large central alveole (see note C) whose thickened 

 wall encloses a globe of jelly and serves as the central support of the whole colony 

 (PI. 5, fig. 1). Still more striking, however, is the arrangement in certain Poly cyttaria, 

 where each individual of the colony (or each central capsule with its calymma) is 

 enclosed in a large alveole, whose firm wall often attains considerable thickness (PI. 4, 

 figs. 2, 3). The whole colony then appears as an aggregate of numerous cells, each of 

 which possesses two envelopes, the inner central capsule and the outer alveolar 

 membrane ; between these lies in the Collosphaerida the siliceous lattice-shell (PL 6, 

 fig. 2). These pericapsular alveoles may be regarded as an outer cell-wall more 

 correctly than the membrane of the central capsule itself, but the arrangement may 

 also be compared to the temporary encystation of other Protista (see note D). 



A. The extracapsular vacuoles in the calymma were first observed in 1851 by Huxley, in 

 Thalassicolla and Sphcerozoum, and compared with Dujardin's sarcode vacuoles (L. N. 5). After- 

 wards J. Miiller noticed that generally these " large clear vesicles are covered by a fine membrane," 

 and hence he called them "alveoles" (L. K 12, pp. 3, 7, &c.). In my Monograph I have described 

 them more in detail as " extracapsular alveoles " (1862, p. 88, Tafs. i.-iii. xxxii.-xxxv.). Ever 

 since then the point has been debated whether these clear spaces are simple vacuoles in the sense 

 of Huxley or vesicular alveoles as stated by J. Miiller. This contention is unnecessary, for both 

 varieties are present, and often no sharp line can be drawn between them. E. Hertwig has recently 

 come to the conclusion that they are as a rule " membraneless vacuoles," but that they " sometimes 

 become surrounded by a special envelope " (L. N. 33, p. 31). He even succeeded " in extracting 

 from a Gollosphcera the large vesicle which lies in the centre of many colonies and removing its 

 covering of central capsules and jelly." 



B. The mechanical importance of the alveolar structure, which certainly increases the elasticity 

 and mechanical resistance of the voluminous calymma, has not yet been sufficiently realised ; in the 

 case of those Radiolaria which have no skeleton, or at all events no lattice-shell, it may takfe the 

 place of this as a protective envelope. Furthermore, by taking in and giving out water it may 

 discharge a hydrostatic function, causing the organism to rise or sink in the water. 



C. The large central alveole found in the colonies of many Polycyttaria (especially Collo- 

 sphterida) and first described in my Monograph (Taf. xxxiv. fig. 1), has since then been observed by 

 Hertwig, Biitschli, and other investigators, and recognised as the " central support of the whole 

 colony, surrounded by a delicate membrane " (compare L. N. 33, p. 31, and L. K 41, p. 436). In 

 a colony of Trypanosphcera transformata (PI. 5 fig. 1), which I observed living while in Ceylon in 

 1881, the membrane of the large central alveole was surrounded by a firm network of sarcoplegma, 

 and could be mechanically isolated from the central jelly-sphere which it enclosed. 



D. The pericapsular alveoles, figured in PI. 4, figs. 2, 3, from a Sphcerozoum, and in PI. 6, fig. 2, 

 from a Siphonosphcera, were very well preserved in some preparations in the Challenger collection ; 

 perhaps their development coincides with the formation of spores, and may be regarded as an 

 encystation. 



