1664 THE VOYAGE OF H.M.S. CHALLENGEE. 



shell are extremely fragile, so that complete and well-preserved specimens are rare. 

 Some species are among the most admirable forms of Radiolaria, and are similar to small 

 elegant Medusae, e.g., Mediisetta (PI. 120, figs. 1-4), Gazelletta (PI. 118, fig. 1), and 

 Gorgonetta (PI. 119). 



The shell of the Medusettida is usually very small and delicate, in comparison to 

 the long, often exceedingly large, feet, which depend from the margin of its mouth. 

 From the size and the weight of these feet, we must suppose that the shell floats 

 in the water in the same position as in the Nassellaeia, so that the main axis is 

 vertical, and the ojrjen mouth with the feet on its lower pole. The form of the shell 

 exhibits the same varieties as the similar umbrella of the Medusae. It is usually more 

 or less hemispherical, sometimes flatter, cap-shaped, at other times more highly vaulted, 

 campanulate or ovate. The similarity with the umbrella of a Medusa is so great, that in 

 many species the large lower opening on the mouth of the shell is surrounded by a 

 prominent ring or diaphragm, comparable to the velum of the Craspedot^ or Hydro- 

 medusa (PI. 120, figs. 3-4). 



The structure of the shell, which we term shortly " alveolate," is very remarkable, 

 and seems to be diff'erent from that of all the other Ph^odaria. Its reticulate appear- 

 ance seems to indicate at first sight the usual lattice-shell, pierced by numerous very 

 small pores (PI. 118, figs. 1—3). But as soon as we make the shell dry, air always 

 enters into its thin walls, and each apparent pore is found to be a small alveole or a 

 separate compartment, which contains a small bubble of air (PL 120, figs. 11, 16). 

 The thin wall of the shell is therefore double, composed of two parallel, very thin lamellae 

 of silica, which are little distant from one another, and are connected by a network of 

 small rods or septa. This network is rarely regular, with square meshes (PI. 118, fig. 3), 

 or with hexagonal or triangular meshes (PL 116, fig. 2); it is usually ii-regular, with 

 polygonal meshes (PL 120, figs. 11—16). The size of the meshes or alveoles is usually 

 smallest on the aboral apex of the shell, and increases gradually towards the mouth. 

 The largest alveoles are around the mouth, on the velum-shaped peristome, and here 

 the marginal alveoles of the shell pass directly over into the separate alveoles or hollow 

 compartments of the articulate feet (PL 118, fig. 1). Probably each alveole is a separate 

 compartment, filled up by jelly, and has a small opening or pore on its inside ; this 

 pore is very distinct in the marginal alveoles of some larger species (PL 116, fig. la ; 

 PL 120, fig. 14), but I have not been al)le to recognise it everywhere. Moreover, it 

 often happens that the alveoles are apparently completely closed. In some smaller 

 species the alveolate structure seems to be present only in the peripheral part of the 

 shell, whilst the central part is solid. 



The outer convex surface of the shell is usually smooth, sometimes papillate 

 (PL 116, fig. l), at other times studded with small, conical, radial spines (PL 118, 

 fig. 1 ; PL 120, figs. 4-11). Prolonged cylindrical spines or tubes are rarely 



