ELEMENTARY VITAL PHENOMENA 



149 



cases the shells are composed of one definite material (Fig. 49). 

 Thus, forms occur that employ only cases of the silicious Algce or 

 diatoms, whileothers em ploy only sand-grains of certain sizes, and still 

 others particles of mud. It has been thought that the Difflugim 

 select their material from substances at their command. But it 

 can be proved, at least in some cases, that no real selection exists 

 here in the same sense as in food-ingestion by the above-men- 

 tioned cells. The fact that forms from one and the same locality 

 employ only a certain material depends rather upon the circum- 



Fio. 49. Various Difflugia-shells, constructed of : A, diatom-cases ; B, fine sand -grains ; C, fine and 

 coarse sand-grains ; J), diatom cases and sand-grains ; , coarse sand-grains ; F, the same form 

 as E, but made of splinters of blue glass. 



stance that in the given locality only this one material is at hand. 

 If, e.g., the dwelling-place of the form that constructs its shell out 

 of mud or substance excreted from its body be examined, it is 

 found that here other materials, perhaps diatom-cases or sand- 

 grains, are wholly wanting. If, however, such forms be given the 

 possibility of getting other material, by the introduction of very 

 finely pulverised sand or, still better, very finely ground, coloured 

 glass into the culture-vessel in which they live, it is found that the 

 individuals arising by reproduction surround themselves with a 

 delicate shell of sand or splinters of coloured glass. 1 The circum- 

 1 Cf. Vervvorn ('90, 1). 



