OF THE DIATOME^. 41 



elevations. So of stripes, costoe, and pinnules : to some, such markings in 

 special instances are ridges ; to others, furrows or fissures ; to others, ele- 

 vations ; and to others again, canals. The ardent microscopical research of 

 this period is daily diminisliing the number of these enigmas, and intro- 

 ducing certainty in place of doubt and vague conjecture. To Ehrenberg's 

 apprehension, many puncta were real pores, and many striae or costse real 

 fissures ; the former of these were supposed to give exit to a few or to multi- 

 tudinous imaginary ' pedal organs ' for locomotion, the latter to serve for the 

 passage of ova, and generally to bring the presumed internal animal organiza- 

 tion into immediate relation with the external medium around it. 



Perhaps the discussion respecting the nature of apparent pores has been 

 most animated in the case of the genera Navicula and Pinnularia, which 

 present a large rounded spot at each extremity of the frustule and a central 

 space known as the umbilicus*^; with a tubular or canal-like band connecting 

 them together (XII. 15, 21, 46 ; XYI. 1). Prom the umbihcus, Ehrenberg 

 believed a single locomotive organ to proceed — an undivided sole-like foot, 

 similar to the locomotive organ of snails, whilst he represented the terminal 

 jDoints to be orifices for the purposes of nutrition (IX. 134). Although 

 denjdng the offices assigned them by the Berlin micrographer, Kiitzing coin- 

 cided with him in the belief of their being actual pores, and supposed that 

 they give exit to a gelatinous substance, such as is actually found sur- 

 rounding some Navkulce, and becomes a prominent character in the tribe of 

 Diatomeae represented by ScJiizonema. Schleiden (Princijyles of Botany, hy 

 Lanhester, p. 594) speaks of the longitudinal band as a cleft, and of the 

 median and terminal spots as circular enlargements or thickened spots of 

 silicious matter. He moreover appends an enlarged lateral view of a Pin- 

 nularia (XYI. 2, 3, 4, 5), to prove that the seeming central orifice is simply 

 a depression. This explanation of their nature coincides in the main with 

 that given by Prof. Smith, who asserts that these markings are due to a lon- 

 gitudinal band of condensed and more solid silex, widened into small expan- 

 sions at the centre and extremities, or at the extremities only, and probably 

 designed to give firmness to the valve. " That these expansions (he adds) 

 are not perforations in the valve, as alleged by Ehrenberg, and acquiesced in 

 by Kiitzing, might be shown in various ways. The internal contents of the 

 frustule never escape at such points when the frustule is subjected to pressure, 

 but invariably at the suture or the extremities .... nor does the valve when 

 fractured show any disposition to break at the expansions of the central Hne, 

 as would necessarily be the case were such points perforations, and not 

 nodules. Moreover, the central band of silex is itself frequently traversed by 

 a narrow line which arises from the confluence of a series of cells, which 

 thus form a minute tube ; but this tube invariably ends in a rounded extre- 

 mity at the central and terminal nodules, and does not pass into an opening 



or aperture in the valve The bending down of this tube and the 



thickening downwards of the silex at the nodules give the semblance of 

 depressions to the siu-face of the valve at such places. But I am disposed to 

 think that this is merely an optical appearance, and at aU events assured 

 that no perforation exists at such points, and that the terms apphed to these 

 nodules by difi'erent authors, implying that they are openings or ostiola, are 

 altogether inadmissible." 



Examples of nodules at the centre and extremities are found in the 

 ^GneTQ. Ampliiprora, Pinnularia, Navicula (XII. 5, 6), and Gomphonema (XII. 

 15, 21, 46). In Stauroneis the central nodule is developed transversely, so 

 as to form a smooth transverse band or ^'stauros" free from markings 

 (XII. 7, 8, 18). A median longitudinal ridge or band exists in Navicula, 



