1 32 COMPARATIVE ANATOMY 



jected, together with other non-nutritious substances which 

 have been taken in, as it were, accidentally. There is no special 

 aperture for the ejection, as there is none for the ingestion, of 

 solid matter. One may say that the Amoeba flows away from 

 its indigestible contents, leaving them behind. 



Since Amoebae live in muddy and sandy bottoms amidst all 

 sorts of inorganic non-nutritious grains and fragments, it is 

 clear that they must exercise some sort of selection in ingest- 

 ing foreign material ; for if they did not, they would take in 

 every particle of convenient size that they meet, and they 

 do not. Moreover, an Amoeba living in water which contains 

 numerous diatoms of relatively large size may be seen to 

 engulf a number of these apparently inconveniently large 

 objects almost to the exclusion of other matters, thus making 

 a distinction between things good for food and things not 

 good. But its powers of discrimination are not great, for it 

 will sometimes ingest grains of quartz sand, and such non- 

 nutritious substances as powdered carmine, or powdered 

 litmus. None the less we must credit the animal, lowly as it 

 is, with a certain power of selection which constitutes a part 

 of its sensibility or irritability. 



In common with all living things, the Amoeba exhibits that 

 response to external stimuli which we call irritability. Thus 

 its movements are retarded by cold, and up to a certain point 

 are made more active by heat. But if the temperature of the 

 water in which it is contained is raised above this point, its 

 movements become less active and cease altogether between 

 30 and 35 C., but begin again when the temperature is 

 lowered. At about 40 C. the protoplasm is coagulated, and 

 the Amoeba is killed. The organism is also responsive to 

 electrical stimuli. If an Amoeba is placed in a drop of water 

 between the electrodes of an induction battery, and a weak 

 shock is passed, it may be seen to withdraw its pseudopodia 

 and contract itself into a ball, gradually recovering its shape 

 and resuming its pseudopodial movement after the effects of 

 the shock have passed away. There is also evidence that 

 Amoebae are sensitive to light. Pelomyxa palustris has been 

 observed to avoid the light, creeping under the mud at the 

 bottom of the vessel in which it was contained during the day, 

 and crawling out on the sides of the vessel during the night. 

 In these diurnal migrations the animalcule traversed a dis- 



