72 Transactions of the Royal Microscopical Society. 
flagellum of Bacterium was revealed to human gaze, through the 
untiring perseverance and skill of Messrs. Dallinger and Drysdale ; 
and to base on this a confident expectation that it is but a mere 
question of time, and the proper use of the all but perfect optical 
appliances which our foremost opticians now place at our disposal, 
when the motile and prehensile filaments of some of the naviculoid 
diatoms, and the elastic investiture of the mysterious Bacillaria 
paradoxa, shall be also revealed to us.* 
There is, however, another point connected with this subject 
that demands careful notice. Dr. Carpenter, in common, I believe, 
with every other writer on the Diatomacex, down to a very recent 
period, regards the so-called “hoop” as a single solid piece formed 
“ by the consolidation of the portion of the cell-membrane which is 
left exposed when the valves separate from each other during the 
development of the new valves.” t 
Now, unless the hoop be really single (which it certainly is 
not), it is evident that, in order to account for the admitted length- 
ening and simultaneous increase in the overlapped parts which take 
place in many of the larger filamentous genera after the new half- 
frustules are fully developed, it would be necessary to assume that, 
altogether externally to the exposed and consolidated surface of the 
inner of the two connecting zones, there must exist, somehow, a 
second exposed surface of “ proper cell-wall,” which in turn becomes 
similarly interpenetrated by silex. It seems difficult to understand, 
however, if this second layer were thrown out, say in the shape of 
a fold or projected process of the “ proper cell-wall,’ how it could 
be that when once fairly silicified, this could admit of the least 
movement either backwards, forwards, or sideways, over the already 
formed inner siliceous layer; unless by taking a third bold leap in 
the dark, and declaring that, as soon as consolidation was secured, 
it becomes a detached but nevertheless integral part of the frustular 
structure. 
Dr. Macdonald says (loc. cit. ante, p. 8):—“ Dr. Wallich seems 
to consider the hoops of the connecting zone quite supplementary 
and not essentially persistent parts of the valves themselves, though 
often easily separated.” 'This—Dr. Macdonald will, I know, excuse 
me for saying—is rather more than I said, and a good deal more 
than I, at all events, meant. In speaking of the two zones as 
deciduous, I had no intention of conveying the idea that I regarded 
them as merely supplementary or other than highly important 
* Moreover, if analogy goes for aught, we have analogous extra-cellular 
structure ready made to hand, as it were, in the extremely delicate sarcodic 
investiture of the foraminiferous shell, which was discovered some five-and- 
twenty years ago by Professor Williamson, of Owen’s College, and which I have 
myself detected as being invariably present in the Polycystina, Acanthometra, 
Dictyochide, and other testaceous oceanic Rhizopods. 
+ Carpenter ‘On the Microscope.’ 1875 ed. p. 307. 
