developed in the following genera; _Thalassicolla_ (Pl. 1, figs. 4, 5), _Thalassophysa_, _Thalassoplancta_, _Lampoxanthium_ (Pl. 2, figs. 1, 2); among the PHÆODARIA in most genera of the Phæodinida, Cannorrhaphida and Aulacanthida (Pls. 101-104), and probably also in other voluminous PHÆODARIA (_e.g._, #Phæosphæria#). The alveoles or vacuoles in the calymma of these large Radiolaria lie usually in several layers, one above another, and increase in size from within outwards. The Polycyttaria or social Radiolaria (the three families Collozoida, Sphærozoida and Collosphærida) without exception have an alveolar structure, and the special form of {lv}their colonies or coenobia is to a great extent determined by the development, number, size and arrangement of the alveoles in their calymma (compare Pls. 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 (Pl. 5, fig. 1). Still more striking, however, is the arrangement of certain Polycyttaria, 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 (Pl. 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 Collosphærida 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 _Sphærozoum_, and compared with Dujardin's sarcode vacuoles (L. N. 5). Afterwards J. Müller noticed that generally these "large clear vesicles are covered by a fine membrane," and hence he called them "alveoles" (L. N. 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. Müller. This contention is unnecessary, for both varieties are present, and often no sharp line can be drawn between them. R. 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 _Collosphæra_ the large vesicle which lies in the centre of many colonies and removing its covering of central capsules and jelly."