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Bacitlaria jparadoxa ; and lastly, of a flattened, quoit-shaped, 
hyaline expansion, surrounding the periphery of each valve, as 
in the beautiful oceanic Diatom, Coscinodiscus sol. Many more 
forms of these appendages might be enumerated differing as 
widely from each other as the above, not only in shape but in the 
purposes they apparently serve in the economy of the several 
species which possess them. I presume, however, that enough has 
been advanced to substantiate what I contend for — namely, that it 
is illogical to regard all these widely varying phases of this extra- 
frustular secretion as unconnected with some more specialized 
function in the protoplasm of the Diatom than has hitherto been 
ascribed to it. The different forms assumed cannot be for- 
tuitous. Their permanence in certain species, and in these 
only, at once negatives any such idea. For the same reason it 
is hardly possible that they can be epiphytic or parasitic growths, 
which have nothing to do with the structure with which they 
are associated. Their perishable nature the moment the parent 
structure dies would almost seem to prove that they cannot be 
looked upon as absolutely effete matter whilst constituting an 
integral part of the living organism. The siliceous wall of the 
Diatom may be dead matter, just as dentine is, though still form- 
ing part of the living animal. But both these portions of 
structure are almost imperishable under ordinary conditions. 
There is, therefore, no analogy between the cases. 
Of the movements observable in. certain Diatoms much might 
be said that bears in an important degree on the present subject. 
For the present I must, however, rest content with repeating the 
conviction expressed by me nearly twenty years ago, in the 
“Annals,” and again very recently in the “Monthly Microscopical 
Journal,” that so palpably are these movements due to some 
subtle form of motile filaments that I do not hesitate to say it 
is but a question of time and properly directed skill when the 
nature of the organ, whatever its exact form may be, shall be 
revealed by the microscope, just as the long-suspected flagellum 
of one of the Bacteria has already been. 
It only remains for me to add that, taking all the forthcoming 
facts into consideration, many of the most material of which my 
limits have precluded me from even naming in the present 
article, there seems to be ample ground for concluding that the 
Desmid and the Diatom, although undoubtedly presenting 
“ closed-cells ” within their structure, are not themselves mere 
cells consisting only of the parts, or homologues of the 
parts, which enter into the composition of the typical vege- 
table cell as ordinarily defined ; but are, in reality, composite 
structures of which the cellular portion constitutes a small, 
though no doubt a very important, integral part. 
