MICROSCOPIC FORMS OF VEGETABLE LIFE. 275 



of a Pecten, or other bivalve shell, along a line of junction or suture; 

 and as each valve is more or less concavo-convex, a cavity is left between 

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

 ity, however, varies widely in different Diatoms; for sometimes each valve 

 is hemispherical, so that the cavity is globular; sometimes it is a smaller 

 segment of a sphere resembling a watch-glass, so that the cavity is len- 

 ticular; 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 with any modifications of the contour of the valves, 

 which may be square, triangular (Fig. 164), heart-shaped (Fig. 176), 

 boat-shaped (Fig. 175, A), or very much elongated (Fig. 171), and may 

 be furnished (though this is rare among Diatoms) with projecting out- 

 growths (Figs. 182, 183). Hence the shape presented by the frustule 

 differs completely with the aspect under which it is seen. In all instances, 

 the frustule is considered to present its ' front ' view when its suture is 

 turned towards the eye, as in Fig. 175, B, c; whilst its ' side ' view is 

 seen when the centre of either valve is directly beneath the eye (A). Al- 

 though the two valves meet along the suture in those newly-formed frus- 

 tules which have been just produced by binary subdivision (as shown in 

 Fig. 167, A, e), yet as soon as they begin to undergo any increase, the 

 valves separate from one another; and by the silicification of the cell- 

 membrane thus left exposed, a pair of hoops is formed, each of which is 

 attached by one edge to the adjacent valve, while the other edge is free. 

 As will be presently explained, one of the valves is always older than the 

 other; and the hoop of the older valve partly incloses that of the younger, 

 just as the cover of a pill-box surrounds the upper par x t of the box itself. 1 

 As the newly-formed cell increases in length, separating the valves from 

 one another, both hoops increase in breadth by additions to their free 

 edges; and the outer hoop slides off the inner one, until there is often 

 but a very small ' overlap.' As growth and self -division are continually 

 going on when the frustules are in a healthy vigorous condition, it is rare 

 to find a specimen in which the valves are not in some degree separated 

 by the interposition of the hoops. 



275. The impermeability of the silicified casing renders necessary 

 some special aperture, through which the surrounding water may come 

 into relation with the contents of the cell. Such apertures are found 

 along the whole line of suture in disk-like frustules; but when the Dia- 

 tom is of an elongated form, they are found at the extremities of the 

 frustules only. They do not appear to be absolute perforations in the 

 envelope, but are merely points at which its siliceous impregnation is 

 wanting; and these are usually indicated by slight depressions of its 



1 This was long since pointed out by Dr. Wallich in his important Memoir on 

 the 'Development and Structure of the Diatom- valve' ("Transact, of Microsc. 

 Soc.," N.S., Vol. viii., 1860, p. 129); but his observation seems not to have at- 

 tracted the notice of Diatomists, until in 1877 he called attention to it in a more 

 explicit manner ("Monthly Microsc. Journ.," Vol. xvii., p. 61). The correctness 

 of his statement has been confirmed by the distinguished American Diatomist, 

 Prof. W. Hamilton Smith; but as it has been called in question by Mr. J. D. Cox 

 ("American Journal of Microscopy," Vol. in., 1878, p. 100), who asserts that in 

 Isthmia there are three hoops two attached to the two valves, and the third 

 overlapping them both at their line of junction, the Author has himself made a 

 very careful examination of a large series of specimens of Isthmia and Biddul- 

 phia, the result of which has fully satisfied him of the correctness of Dr, Wai- 

 lich's original description. 



