MICROSCOPIC FORMS OF VEGETABLE LIFE. 303 



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 particles of mud on the other hand, will 

 float after the Diatoms have subsided. The tendency of living Dia- 

 toms to make their way towards the light, will afford much assistance in 

 procuring the free forms in a tolerably clean state; for if the gathering 

 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. Marine forms must be looked-for upon Sea-weeds, and in the 

 fine mud or sand of soundings or dredgings; they are frequently found 

 also, in considerable numbers, in the stomachs of Holothuriae, Ascidians, 



Fossil Diatomacece, etc., from Oran: a, a, a, Coscinodiscus; 6, 6, 6, Actinocyclus; c, Dictyochya 

 fibula; d, Lithasteriscus radiatus; e, Spongolithis acicularis; /, /, Grammatophora parallela (side 

 view) ; g, g, Grammatophora angulosa (front view). 



and Salpae, 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 Dia- 

 tom-collector will do well to examine the digestive cavity of any small 

 aquatic animals that may fall in his way; rare and beautiful forms having 

 been obtained from the interior of Noctiluca (Fig. 297). The separation 

 of the Diatoms from the other contents of these stomachs must be accom- 

 plished 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 sedi- 

 ment then allowed to subside for some hours before the water is poured 

 off, since, if it be decanted too soon, it may carry the lighter forms away 

 with it. Some kinds of earth have so little impurity that one washing 

 . p ^^oo: hv.t in p.nv or:?o i 4 : i 5 ? to Ve continued so long as the water remains 



