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 take 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 Collosphærida) and first described in my Monograph (Taf. xxxiv. fig. 1), has since then been observed by Hertwig, Bütschli, 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. N. 41, p. 436). In a colony of _Trypanosphæra transformata_ (Pl. 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 Pl. 4, figs. 2, 3, from a _Sphærozoum_, and in Pl. 6, fig. 2, from a _Siphonosphæra_, 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. {lvi}87. _The Extracapsular Fat-Globules._--Fat is probably as widely distributed in the exoplasm as in the endoplasm of the Radiolaria; a considerable proportion of the small, dark, highly refractive granules appear to consist of fat; most likely they are for the most part direct products of metastasis. These widely-spread granules, which are sometimes coloured, and which by their passive motion produce the phenomenon of granular circulation in the exoplasm, are not the only fatty structures in the extracapsulum; larger globules sometimes occur. In certain large #Collodaria# (_e.g._, _Thalassicolla melacapsa_, Pl. 1, fig. 5; _Thalassophysa sanguinolenta_, &c.) radial series of oil-globules are found in the calymma, especially in its proximal portion; in others the central capsule is surrounded by a layer of oil-globules (situated in the sarcomatrix). In the PHÆODARIA a part of the phæodium appears to consist of fat-globules.