72 I Transactions of the Society. 



II' this valve, formed of a siliceous frame-work arranged in the 

 fashion of a honeycomb, with its intricate system of perforations 

 serving the purpose of a filter, has to be conceived as a congeries 

 of polygonal cells, each with a very finely divided cap on the top, 

 and a still more finely perforated membrane at the bottom of the 

 cell, what is the ultimate fate of the bacterial foes imagined by 

 Mr. Nelson as attacking the diatom ? When strained off, are they 

 detained within the cell ? Are they removed sooner or later by 

 some system of scientific backwash not yet outlined, or are they 

 imprisoned for life to act as a deposit of microscopic sludge on 

 the floor of the cell, thus preventing the access of the very medium 

 necessary to the life of the diatomic organism under consideration ? 

 Such an example of scientific Hari-Kari, of methodically deliberate 

 race suicide, is out of the question. 



Series IV. A. and B. — Though one is in doubt as to what the 

 new lens has shown Mr. Nelson in the already well-observed slide 

 that has been in his test box since 187G,* the photographs in 

 Series IV. show quite convincingly that naturally the opening in 

 the "eye-spot layer" is clear and unobstructed, so that it may 

 perform its unmistakable duty of allowing the filtered water free 

 access through the filter cell to the organism within the frustule. 



In each group of the series separate fractures are dealt with. 

 In Group A finally the broken edges are seen to be sharp and clean 

 cut, without a trace of crenation comparable to that seen at the 

 edge of the perforated cap (Photo 2, Series II.), and the appearance 

 is exactly what one would accept it as being — a broken edge of the 

 short tube-like cell, described by Mr. H. Morland as giving " a ringed 

 appearance to the perforations when the valve is examined direct 

 on the inner surface." 



In Group B the mounter's bristle has, I believe, pierced the 

 surface of the valve, lifted off a portion of the crust covering several 

 of the polygonal cells, leaving fortunately the bottom of the cells 

 exposed and intact as an " eye-spot layer." The photographs of the 

 last resolutions of this floor show corroboratively that the openings 

 are clear and unobstructed, and the series is offered as proof from 

 the optical point of view that the probability of the presence of 

 even a " diaphanous " membrane, first " glimpsed " years ago, and 

 now, on the advent of the new lens, resolved into perforations by 



* See this Journal, 1910, p. 147. 



EXPLANATION OF PLATE XXVI. 



Pig- 1.— Outer veil of siliceous meshwork. x 2700. 

 „ 2. Ditto, x 1000. 

 )> 3. — Fragment of the polygonal cell-layer, with the " ringed " openings 



of Morland. x 1000. 

 „ 4. Ditto. Ditto. 

 „ 5. Ditto. Ditto. 



