NERVOUS SYSTEM AND SENSE-ORGANS 



studying a very simple organism, such as the Proteus Animal- 

 cule (Amceba}, which is a particle of comparatively pure proto- 

 plasm (fig. 1006). That this creature is sensitive to mechanical 

 stimuli is easily proved by tapping the glass slide on which one 

 is crawling about under the microscope. The protruding lobes 

 of the body (pseudopods) by which creeping is effected will be 

 drawn in, and the animal will assume a spherical form (fig. 1006, A). 



A good example of chemical stimu- 

 lation is afforded by the addition of 

 very weak caustic potash to the 

 water in which the slug -shaped 

 species of Amceba (A. Umax) is 

 moving along, the reaction consisting 

 in this case of the protrusion of long 

 pointed pseudopods (fig. 1006, B). 

 The same kind of Amceba reacts in 

 a marked way to changes of tem- 

 perature (fig. 1006, c). At freezing 

 point (o C.) it is spherical and inert; 

 as the temperature increases from 

 o C. to 35C. it moves about with 

 ever -increasing activity; above this 

 the activity gradually diminishes; 

 and at about 40 C. the animal has 

 assumed a spherical form, and dies 

 in a condition of " heat -stiffening" 

 or coagulation. This illustrates very 

 well the fact that any particular 

 stimulus has only a certain range of 

 action, the range in this case being 

 between o e C. and 40 C., which are known as the minimum 

 and maximum points of heat-stimulation for this particular animal. 

 Between these two points is an optimum one (35 C. in this 

 instance), at which the stimulus exerts its greatest effect by 

 way of promoting activity. Light does not appear to affect 

 the creeping movements of Amceba, but is said to check the 

 taking in of food, which process goes on most actively at night. 

 If a constant current of electricity is passed through the body 

 of an Amceba which is protruding pseudopods in all directions 

 it will begin to creep against the current, and all those pseudo- 



Fig. 1006. Proteus Animalcules (Amceba], 

 much enlarged, showing effect of various stimuli, 

 i and 2, Appearance before and after application 

 f stimulus. See text. 



