314 COLLECTION OF DIATOMACEjE. 



distinguishable as white or cream-coloured powders of extreme 

 fineness. 



237. For collecting fresh Diatomacea?, those general methods 

 are to he had recourse to which have been already described 

 (§§176,208). " Their living masses," says Prof. W. Smith, 

 1 ' present themselves as coloured fringes attached to larger plants, 

 or forming a covering to stones or rocks in cushion-like tufts — or 

 spread over their surface as delicate velvet — or depositing them- 

 selves as a filmy stratum on the mud, or intermixed with the scum 

 of living or decayed vegetation floating on the surface of the water. 

 Their colour is usually a yellowish-brown of a greater or less in- 

 tensity, varying from a light chestnut, in individual specimens, to 

 a shade almost approaching black in the aggregated masses. 

 Their presence may often be detected without the aid of a micro- 

 scope by the absence, in many species, of the fibrous tenacity 

 which distinguishes other plants : when removed from their natural 

 position they become distributed through the water, and are held 

 in suspension by it, only subsiding after some little time has 

 elapsed." Notwithstanding every care, the collected specimens are 

 liable to be mixed with much foreign matter : this may be partly 

 got rid of by repeated washings in pure water, and by taking 

 advantage, at the same time, of the different specific gravities of 

 the Diatoms and of the intermixed substances to secure their 

 separation. Sand, being the heaviest, will subside first ; fine par- 

 ticles of mud, on the other hand, will float after the Diatoms 

 have subsided. The tendency of the Diatomaceai to make 

 their way towards the light will afford much assistance in pro- 

 curing the free forms in a tolerably clean state ; for if the gather- 

 ing which contains them be left undisturbed for a sufficient length 

 of time in a shallow vessel exposed to the sunlight, they may be 

 skimmed from the surface. The Marine forms must be looked for 

 upon Sea-weeds, and in the fine mud or sand of soundings or dred sw- 

 ings ; they are frequently found also, in considerable numbers, in 

 the stomachs of Holothurise, Ascidians, and Salpte, in those of the 

 oyster, scallop, whelk, and other testaceous Mollusks, in those of 

 the crab and lobster, and other Crustacea, and even in those of the 

 sole, turbot, and other ' flat-fish.' In fact, the Diatom-collector 

 will do well to examine the digestive cavity of any small aquatic 

 animals that may fall in his way : rare and beautiful forms have 

 been obtained from the interior of Noctiluca (Fig. 282). The sepa- 

 ration of the Diatoms from the other contents of these stomachs 

 must be accomplished by the same process as that by which they 

 are obtained from Guano or the calcareous Infusorial Earths ; of 

 this, the following are the most essential particulars. The guano 

 or earth is first to be washed several times in pure water, which 

 should be well stirred, and the sediment then allowed to subside 

 for some hours before the water is poured off, since, if it be de- 

 canted too soon, it may carry the lighter forms away with it. 



