329 



DIATOMACE^E. 



DIATOMACEvE. 



330 



of the epidermal valves aud the formation of the siliceous connecting- 

 membrane already described. In the centre of the enlarged frustule, 

 in exact apposition to the original valves and closely applied to them, 

 there are now found two new valves, covering the surface of the cell- 

 membranes along the line of fission. The divided portions of the 

 endochrome spread themselves along the membrane which is embraced 

 by the new valves, and there result two half-new frustules bound 

 together by the connecting-membrane, generated during the process 

 we have described. 



" During the healthy life of the Diatom the process of self-division 

 is being continually repeated ; the two half-new frustules at once pro- 

 ceed to divide again each into two frustules, and thus the process con- 

 tinues. I have been unable to ascertain the time occupied in a single act 

 of self-division, but supposing it to be completed in twenty-four hours, 

 we should have, as the progeny of a single frustule, the amazing num- 

 ber of one thousand millions in a single month ; a circumstance which 

 will in some degree explain the sudden or at least rapid appearance of 

 vast numbers of these organisms in localities where they were but a 

 short time previously either unrecognised or only sparingly diffused." 

 (Smith, p. 25.) 



1. Etinotia Diadcma. 2. Eupodiscut tculptus. 3. Tricerntium Faros. 4. 

 Burirella biteriata. 5. Synedra gracilis. 6. A'avicula elegant. 7. J'leurosigma 

 anguttatvm. 8. Qocconema lanceolntum t portion representing conjugation. 

 9. Gomphonema geminatum. 10. Meridion circulate. 11. Jtacillaria para- 

 (torn. 12. Acttnanthes longipts. 13. Striatclla impunctata. 14. Diatoma 

 tvlgaie, the frustules united. 15. Biddulphia pulchella. 16. Melosira variant ; 

 the enlargement is peculiar to the genus ; a, side view. 17. Dickieia ulvoidet ; 

 a, frnstule; h, frond. 18. Schixotiema Smithii ; a, frustule; b, ditto; c, 

 frond, natural size; rf, frond magnified. 19. Epithemia tvraida, illustrating 

 the process of conjugation. From a drawing by Mr. West. 



The structure of the siliceous portion of the Diatomacea is the most 

 remarkable part of their organisation. The following is Meneghini's 

 account of this organ : 



" Kvery Diatom is formed of a siliceous shield and a soft substance 

 therein, contained. According to Kutzing, this shield consists of pure 

 silica, or, in some cauea, perhaps, of silica combined with alumina. 



Niigeli further says that the silica is deposited in the outside of au 

 organic membrane, which be believes to be of a vegetable nature. In 

 fact, an organic membrane ought to exist, for the silica could not 

 become solid except by crystallising or depositing itself on some 

 pre-existing substance. On the other hand, we cannot admit, with 

 Niigeli, that it has been deposited externally ; for in many genera, 

 and especially in the Achnanthidia, the siliceous shield is covered 

 with a very delicate dilatable membrane, itself containing silica, as 

 is proved by its sustaining unchanged the action of fire and acids. 

 Therefore, comparing this shield with other organic formations, 

 whether animal or vegetable, containing in like manner either silica 

 or some other so-called mineral element, we may reasonably consider 

 it to be formed of an organic tissue permeated by silica. This 

 permeation may occur either in the wall of a simple cell, as is seen 

 in the epidermal cells of many plants, or within minute cells, as in 

 various plants and animals. The action of heat or of acid, in these 

 cases, destroying the organic matter and leaving the silica untouched, 

 does not alter the apparent form of the organ, because the skeleton 

 remains unaltered. 



" Externally to the shield Kutzing observed a thin stratum which 

 he denominated cement, which may be made visible either by desic- 

 cation or by calcination ; and produces either a simple opacity, or 

 lines, points, and macula;, sometimes irregularly disposed, sometimes 

 regularly. He supposes it to be a silicate of iron or of alumina. 

 Independently of the chemical materials which it may contain, this 

 outside integument seems to me the more important inasmuch as even 

 without resorting to the means indicated by Kutzing, I observe it to 

 be constant, not merely in the species enumerated by him, but also 

 in many others, and I could almost assert that it exists in all. For to 

 me it appears to correspond with that fine membrane of the Achnan- 

 thidia above mentioned, which, according to Kiitzing's own observa- 

 tions, is always visible whenever the two new individuals (into which 

 every Diatom is resolved in its multiplication by deduplication) 

 (sdoppiamento) begin to separate. The lines and points supposed to 

 belong to the subjacent shield belong very frequently to this kind of 

 covering. 



" The shield itself is formed of at least four pieces, or valves, 

 united together in a four-sided figure a tetragon. The mode of 

 union is unknown. But the existence of a kind of articulation 

 which permits an opening and closing, like the valves of a shell-fish 

 described by Corda in a species of Surirella, has been denied by other 

 observers. Be this as it may, whether spontaneous after death or 

 induced by external means, this separation does take place in a regular 

 manner. Now, if we suppose an organitf cell with a wall permeated 

 by silica, and with a four-sided figure, we can easily suppose that all 

 the sides will mechanically support each other. Moreover, we shall 

 meet with numerous facts by a different kind of analogy, namely, that 

 with solid animal tissues belonging either to the internal skeleton or 

 the external tegument. 



" The four valves are equal in length, but in many species and genera 

 one pair exceeds the opposite pair in breadth. In order to establish 

 an uniform language it is convenient to term those primary valves or 

 surfaces which exhibit along the middle the line of division in the 

 act of deduplication, which, since it is formed here in a normal 

 manner, runs parallel to the other two surfaces, denominated lateral. 

 Along the primary surfaces we frequently see longitudinal lines, which 

 terminate at the two extremities in small apertures. From their in- 

 ternal surface there project into the cavity linear marks variously 

 formed but always longitudinal : these are termed vittse. 



" The lateral surfaces have frequently a round aperture of greater 

 or smaller size in the centre, and from this a fissure extends towards 

 each extremity. This fissure either loses itself gradually or expands 

 into the regular terminal apertures. When this occurs each of these 

 surfaces is divided into two distinct valves. On these lateral surfaces 

 we observe the striae, lines, and transverse costa:, no less admirable 

 for their beautiful appearance than for their constant regularity in 

 number, direction, and proportion. When many individuals are 

 united together to form one compound being, like a polyp for instance, 

 it is always by the lateral surfaces that they touch each other; and 

 since all other characters sometimes fail, we can affix to them the 

 denomination 'lateral' from this principal one. 



" Besides the vittse before mentioned, in some genera (Biddulphia,, 

 fig. 15, Climacosphenia, Terfsinoi) there are other solid substances in 

 their internal cavities : these are variously arranged. 



" These essential peculiarities of the shield may perhaps be regarded 

 as indicating a complex structure, very different therefore from what 

 would be prescribed by a simple cellular wall. Ehrenberg deduces 

 from it an argument to compare it with the shell of Molliixca. The 

 Anellinte may be cited among the fnfusoria. Kutzing states, in reply, 

 that among vegetable cells there is found a peculiar conformation of 

 the walls, with prominences, depressions, points, lines, papillce, and 

 perforations, disposed in a regular manner; he refers to grains of 

 pollen, as an instance. He might have added the more appropriate 

 instance of the Desmidiece, which would be very closely allied to the 

 Diatomece, if the latter, like the former, could be referred to the vege- 

 table kingdom. If not equal in constancy and regularity, the 

 Desmidece display a greater degree of complication ; and we must 

 remember the different nature of their substance, for in the vegetable 



