%■ '% 



236 S. O. MAST 



marian and even after it was apparently in the body it was 

 carried out with the neck a considerable distance whenever the 

 head was fully extended (see fig. 5). 



The securing of food in Lacrymaria is strictly on the trial 

 basis. There can be no question but that it has the power of 

 selection in this matter. Inorganic particles are never swallowed. 

 The body of these animals is usually well concealed in debris 

 as already stated, and the anterior end is stretched out in every 

 direction, sometimes to a distance equal to eight times the 

 length of the body and every nook and crevice within this radius 

 is explored. During this process the head is repeatedly extended, 

 jerked back and turned in different directions in rapid succession, 

 giving the appearance of a most active, nervous and exciting 

 search very much like the movements of Didinium (Mast, 1909, 

 p. 96). A strong current clearly seen in solutions having par- 

 ticles in suspension is produced by the action of the powerful 

 oral cilia. This current, especially prominent when the body is 

 fixed, no doubt brings the mouth in close contact with various 

 objects which would otherwise be only lightly touched or escape 

 entirely in the searching movements. Thus it is clearly seen 

 that many particles are tested and a few selected. Indeed, in 

 no protozoan is the power of selection of food more evident 

 than it is in this creature with perhaps the exception of Didinium 

 (Mast, 1909, p. 113). As far as I could ascertain it swallows no 

 inorganic matter at all. Specimens were repeatedly kept from 

 one to forty-eight hours in solutions containing carmine or 

 Chinese ink finely ground, but in no instance were any of these 

 particles found in the animals. Moreover, they evidently also 

 have the ability to discriminate between different living organ- 

 isms of the same size or nearly so, for I have frequently seen 

 many organisms rejected which were plenty small enough for 

 them to swallow. As to the mechanism of selection in this pro- 

 tozoan we are entirely in the dark. Our evidence does not even 

 warrant a conclusion as to whether it is on the basis of chemical 

 or tactile stimuli. However, but little more is known regarding 

 the process of selection of food in other unicellular forms, although 

 Schaeffer (1909, p. 889) thinks that his observations show that 

 the selection of food in Stentor is dependent upon tactile stimu- 

 lation and not upon chemical. 



