136 COMPARATIVE ANATOMY 



almost to the exclusion of other substances, thus making a 

 discrimination 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, or 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 

 mechanical, chemical, thermal, and electrical stimuli. It is 

 possible to draw out a glass rod to so fine a point that it may 

 be used to prick or stimulate any desired part of the 

 Amoeba's body. If such a point be thrust into the advancing 

 end of an Amoeba, the forward movement is at once arrested ; 

 the ectosarc of the region stimulated contracts, but the for- 

 ward streaming movement of the granular endosarc continues 

 for a short time, causing a bulging of the middle of the body. 

 Then the ectosarc is protruded at some point removed from 

 the area of stimulation, the current of granules is reversed and 

 a new pseudopod is formed, and the animal starts off on a new 

 course, generally at right angles to the old one. In this case 

 the stimulus is a strong one, and the Amoeba moves away 

 from it. This is called a negative reaction. But if the 

 stimulus is weak, a positive reaction follows. Thus an Amoeba 

 moving along a surface may be seen to turn towards small 

 objects with which it comes in contact by its advancing end. 

 A negative reaction follows when chemical or thermal stimuli 

 are applied to one side of the body, the phenomenon being 

 essentially the same as in the case of a strong mechanical 

 stimulus. For example, if a minute drop of methylene blue 

 or other chemical is brought into contact with the advancing 

 end of an Amoeba by means of a fine capillary tube, move- 

 ment in that direction is arrested, and a new pseudopod is 



