RELATION OF BURSARIA TO FOOD 43 



facts which have been found in this connection are in agreement 

 with the results and conclusions so far as they have been worked 

 out by Metalnikow ('12) for Paramecium. 



THE RELATION OF BURSARIA TO DIGESTIBLE AND NON-DIGESTIRLE 



SUBSTANCES 



1. The external relations 



Many substances which are in the ordinary sense chemically 

 indifferent to the organism are likewise eaten, though generally 

 in small quantities. Among these are cinnabar, carbon black, 

 Chinese ink, powdered aluminium and the like. The relation of 

 Bursaria to this class of substances is however strikingly differ- 

 ent inside of the cell and to a large extent outside, when com- 

 pared to that relation in the case of digestible and assimilable 

 ones. The fact that some comparatively indifferent substances 

 like the above, are eaten does not affect our conclusion drawn 

 above, as to the paramount importance of the chemical prop- 

 erties of the food in food selection. Chinese ink contains some 

 mucilaginous matter which as my own observations have shown 

 me, is reacted to positively by Bursaria and hence the ink is 

 quite readily eaten. Carmine is a similar substance which though 

 generally taken to be insoluble in water, is in fact sufficiently 

 soluble clearly to affect the feeding reactions of Bursaria. Fur- 

 thermore, the fact that a substance may be insoluble does not, 

 of course, prove that the stimulus from it is not a chemical one, 

 for it is probable, that with such substances as aluminium, cata- 

 lytic or other specific chemical, or even physical reactions depend- 

 ent upon the chemical properties of the substance, are produced 

 by contact with the plasma membranes. The possible variety 

 of interactions of the cell with different kinds of substances when 

 considered in this order of magnitude may of course be very 

 large. 



As regards the eating of non-digestible substances, powdered 

 aluminium may serve, in one way, to illustrate the external rela- 

 tions. If a large number of individuals are put into a suspension 

 of aluminium, often few if any will eat any of the particles of 



