DIATOMACRS. 



DIATOMACE.E. 



,.,.!, un lime or iliea predominates, the wall become* uniform and 

 regular (f) (uniform* od wregoUre)." 



Tb tiliooou* epiderui presents an extraordinary variety of form*, 

 which in every genus and species offer the beet possible means of 

 distinction and identification : (true, or line* frequently moniliform, 

 dote arranged in a radiate or concentric manner, and minute divisions 

 pmentini. perfectly hexagonal outlines, are amongst the moat frequent 

 occurrence*. Great difference of opinion exiU a* to the nature of 

 the**. Mr. Smith *y, " I am diipoeed to regard them all a* modifi- 

 eation* in the arrangement of the rilex of the valve, arising from the 

 mode of development peculiar in each cane to the membrane with 

 which the ailex U combined." He alao denies that there are any 

 perforation* in the valve, a* suppose.! by Ehrenberg and Kutzing. 

 Thene foramina are alao denied by Schleiden. Mr. Smith denies also 

 that the valve* are externally covered with any organic membrane. 



The delicacy of the markings on many of the Diatomacetr render 

 them objects peculiarly adapted for testing the powers of the object- 

 glauea of the microscope. The following table, drawn up by Messrs. 

 Sollitt and Harrison of Hull, to whom microscopy is indebted for 

 having first pointed out this method of testing the powers of the 

 microscope, was presented by them at the meeting of the British 

 Association at Hull in 1853 : 



The Diatomoftit possess the power of moving. " The cells have 

 no special organs tor these movements. But as, in consequence 

 of their nutritive processes, they take in and give out fluid 

 matters, the cells necessarily move when the attraction and the 

 emi.-cion of the fluids is unequally distributed on parts of the surface, 

 and U so active as to overcome the resistance of the water. This 

 motion consequently is observed more particularly in those cells 

 which, in consequence of their taper forms, easily pass through the 

 water ; these cells moreover move only in the direction of their long 

 axis. If one-half of a spindle-shaped or ellipsoidal cell chiefly or 

 exclusively admits material, the other half, on the contrary, giving it 

 out, the cell moves towards the side where the admission takes place. 

 But, as in these cells both halves are physiologically and mor- 

 phologically exactly alike, so it is that it is first the one and then 

 the other half which admits or emits, and consequently the cell 

 moves sometimes in one, sometimes in the opposite direction." 

 (Nageli.) 



This is perhaps as satisfactory an explanation of these movements 

 as can be given in the present state of our knowledge. All observers 

 agree that they can find no evidence to support Ehreuberg'a notion of 

 a pedal or motile organ projected from the interior of the siliceous 

 shield. The Rev. W. Smith has also detected cycloeis in the JHa 

 tomacae. " A distinct movement," he says, " of the granular particles 

 of the endochrome, closely resembling the circulation of the cell 

 contents in Clotlfrium Lunula, noticed by Mr. Ralfs [DESMIUIE^E], and 

 which I have frequently detected in the same species, has occasionally 

 fallen under my notice in some of the larger forms of Diatomacea? 

 He has observed it in SurirtUa bitrriala. (fig. 4.) " This circulation,' 

 he continues, ha* not however the regularity of movement so con 

 picuous in the Dttmidiea, and i* of too ambiguous a character to 

 furnish date for any very certain conclusions, save one, namely, that the 

 Diatom must be a single cell, and cannot contain a number of separate 

 organs, such a* have been alleged to occupy it* interior ; since the 

 odochrome move* freely from one portion of the frustule to another 

 approaching and receding from the central nucleus, unimpeded by 

 aLlinterv,^ obstacle? 



The IHatomaeta are the most abundant and extensively distributed 

 of unicellular organism*. They are found in the ocean, at the mouths 

 of riven, in bnckinh waters, in riven, lakes, {mods, ditches, pools, 

 and cisterns. In fact, wherever a few drop* of water are allowed to 

 remain exposed to the air, we may expect to find form* of Diatomacea 

 Their form* are not Irs* abundant than their presence. In the fin 

 volume of hi* 'Synopsis of the British Diatomacete,' the Rev. W 

 Smith ha* described upwards of 220 species, and the second wil 

 contain nearly 100, so that the number of ipecie* known in Urea 

 Britain is considerably above 300. The facility with which their form 

 are preserved, give to these objects a great advantage, and a bandfu 

 of sand from tie sea or mud from a river in the most remote diatric 

 of the world may be expected to reward the observer with an abund 

 anoe of new forma. They occur in great abundance in the rive 

 Thames, and its mud affords a Urge variety of the frustules of those 

 which have ceased to exist In a report on the ' Microscopical Examin 



tion of the Thames and other Water,' by Urn. Lankeater and Uedfern, 

 pwards of forty species were observed. 



The mode of collecting living specimens for observation is simply 

 o allow the water in which they exist to stand for a few hours, when, 

 >y carefully decanting the water, a portion remains at the bottom of 

 tie vessel more turbid than the rest, and which generally contains in 

 urge numbers the objects sought for. 



In describing showers of coloured dust which have occurred in 

 arious parts of the world, Ebreuberg has demonstrated that various 

 onus of Jsialomacea have been found present In some seasons these 

 rganisuis occur in such uumbeni in the waters of riven as to give to 

 their banks a peculiar physical aspect In the autumn of 1841 the 

 tones and pebbles in the nearly dried-up bed of the Annan, in Dum- 

 riesshire, presented an appearance as though they were white-washed. 

 ?he substance which gave the stones this appearance could be scraped 

 off, and looked like some form of calcareous matter. On submitting 

 his powder to the microscope, Dr. Lankester found that it consisted 

 entirely of the siliceous shields of a species of Synrttra. (Fig. 6.) In 

 he first volume of the new series of the ' Transactions of the Micros- 

 copical Society,' Mr. Shadbolt has given an account of the examination 

 of portions of mud given him by Mr. Busk from Port Natal. This 

 mud was recent, and from the nature of the specimens in it, Mr. Shad- 

 rait thinks it probable that it was obtained hot far from the mouth 

 of some river. In this mud he made out fifty-five distinct species of 

 Diatomacea, twenty of which he has described an entirely new species. 

 Ln the ' Microscopical Journal' for July 1853, Mr. Brightwell of Nor- 

 wich has described nine new species of one genus Trictralium. Six 

 of these are recent Ue says, " We have detected nearly all the 

 recent species described in this memoir in material obtained from the 

 surface of the large sea-shells of the genera Hippopiu and Ilaliotu 

 jcfore they have been cleaned. Many of them in this state are 

 covered with small zoophytes, minute alga;, and other parasites ; and 

 t>y a careful examination of these, Trictratia, and other Diatomactas 

 liave been obtained." 



One of the most singular positions in which Diatoms are found is 

 in the guano brought from America and Africa. Their history is 

 curious. They must first have been swallowed by fish and subse- 

 quently by birds ; their shields however have been able to withstand 

 this double process of digestion, and they are found in large numbers 

 In every pure specimen of guano. Some of the forms which have 

 been thus presented to the naturalist are entirely new, and are amongst 

 the most singular of the family. It has been suggested that the silex 

 thus introduced into the guano may contribute to its fertility, as it is 

 well known that this substance is present in the stems of all our 

 cereal grasses, and U necessary to their growth. 



If they occur thus abundantly in recent deposits, it would be 

 expected that they should be found in many of the older formations 

 of the earth's surface. This is very extensively the case ; although 

 it may be doubted whether, from the fact of their being occasionally 

 found in igneous rocks, that they were amongst the first organisms on 

 the earth's surface. Ehrenberg has been able to detect their presence 

 in some of the earliest rocks of the PaUcozoic series. How Diatomacea 

 may be present in igneous rocks has been suggested by Dr. Hooker. 

 During his voyage with Sir James Ross in the Antarctic Ocean he says, 

 " This order occurred in such countless myriads as to stain the sea 

 every where of a pale ochreous-brown, in some cases causing the 

 surface of the ocean, from the locality of the ships, as far as the eye 

 could reach, to assume a pale-brown colour." This immense mass of 

 organisms perishing are producing a sub-marine deposit, or bank, of 

 vast dimensions, resting on the shores of Victoria Land, and hence on 

 the sub-marine flanks of Mount Erebus, an active volcano upwards of 

 12,000 feet high. " Knowing as we do that Infusoria, Dialomacerr, and 

 other organic constituents, enter into the formation of the pumice and 

 ashes of other volcanoes, and are still recognisable in those minerals, 

 it is perhaps not unreasonable to conjecture that the subterranean and 

 subaqueous forces which kept Mount Erebus in activity, may open a 

 direct communication between this Diatomaceous deposit and its 

 volcanic fires." 



Ehrenberg has described a large number of forms of Diatomacett 

 from the oolite, cretaceous, and other secondary rocks. A formation 

 occurring in Barbadoes, and described by Sir Robert Scbomburgk in 

 the ' Reports of the British Association j for 1847, furnished him with 

 an entirely new group of beings apparently related to this family. 

 The following is Sir Robert's account of this discovery : 



" In the white marls and other rocks of Scotland district, Professor 

 Ehrenberg of Berlin discovered a new and great group of siliceous- 

 shii'lded animalcules, which, in a report read before the Royal Aca- 

 demy of Sciences, he described as Polycyttinu. The regular apertures 

 and articulation of the minute shells which cover these animalcules 

 distinctly bespeak an independent animal structure and development 

 They possess large apertures at the extremity of the body, which has 

 no analogy among plants, but occur very commonly among animals. 

 These siliceous loricated organic forms from the rocks in Barbadoes 

 differ alike from folygatlrica and Polythalamia, but develop an 

 important relation to these two groups, which Professor Ehrenberg 

 considers, not upon conjecture but from actual investigation, to form 

 two separate type*. They approach most nearly in systematic arrange- 

 ment to Polythalamia, and would occupy a separate group among 



