27 G GENERAL CHARACTERS OF DIATOMACE^. 



to form a complete circle, but a spiral of several turns, as in 

 Meridion circulare (Fig. 124). Many Diatoms, again, possess a 

 Stipes, or stalk-like appendage, by -which aggregations of frustules 

 are attached to other plants, or to stones, pieces of wood, &c. ; and 

 this may be a simple foot-like appendage, as in Achnanthes longipes 

 (Fig. 138), or it may be a composite Plant-like structure, as in 

 Lichmophora (Fig. 126), Gomphonema (Fig. 139), and Mastogloia 

 (Fig. 142). Little is known respecting the nature of this stipes; 

 it is, however, quite flexible, and may be conceived to be an exten- 

 sion of the cellulose coat unconsolidated by silex, analogous to the 

 prolongations which have been seen in the Desmicliacece (§ 200), 

 and to the filaments which sometimes connect the cells of the 

 Palmellacece (§ 239). Some Diatoms, again, have a mucous or 

 gelatinous investment, which may even be so substantial that their 

 frustules lie as it were in a bed of it, as in Mastogloia (Figs. 142, 

 143), or which may form a sort of tubular sheath to them, as in 

 Schizonema (Fig. 139). In a large proportion of the group, how- 

 ever, the frustules are always met with entirely free ; neither 

 remaining in the least degree coherent one to another after 

 the process of binary subdivision has once been completed, nor 

 being in any way connected either by a stipes or by a gelatinous 

 investment. This is the case, for example, with Triceratium 

 (Fig. 119), Pleurosigma (Fig. 120), Actinocyclus (Fig. 144, b, b), 

 Actinoptychus (Fig. 120, b, b), Arachnoidiscus (Plate X.), Cam- 

 pylodiscus (Fig. 130), Surirella (Fig. 129), Coscinodiscus (Fig. 

 144, a, a, a), and many others. The solitary discoid forms, 

 however, when obtained in their living state, are commonly found 

 cohering to the surface of Seaweeds. 



214. We have now to examine more minutely into the curious 

 structure of the Siliceous envelope which constitutes the charac- 

 teristic feature of the Diatomaceae, and the presence of which im- 

 parts a peculiar interest to the group, not merely on account of 

 the elaborately-marked pattern which it often exhibits, but also 

 through the perpetuation of the minutest details of that pattern in 

 the specimens obtained from Fossilized deposits (Figs. 144, 145). 

 The siliceous envelope of every Diatomaceous cell or ' frustule ' con- 

 sists of two vcdves or plates, usually of the most perfect symmetry, 

 closely applied to each other, like the two valves of a Mussel or other 

 bivalve shell, along a line of junction or suture ; and each valve 

 being more or less concavo-convex, a cavity is left between the 

 two, which is occupied by the cell-contents. The form of this 

 cavity, however, varies widely in different Diatoms ; for sometimes 

 each valve is hemispherical, so that the cavity is globular ; some- 

 times it is a smaller segment of a sphere resembling a watch-glass, 

 so that the cavity is lenticular ; sometimes the central portion is 

 completely flattened and the sides abruptly turned-up, so that the 

 valve resembles the cover of a pill-box, in which case the cavity 

 will be cylindrical ; and these and other varieties may co-exist 



