84 PHYSIOLOGICAL ACTIVITIES OF THE PROTOZOA 



Oxygen, necessary for the various processes of oxidation, is taken 

 in through the general surface of the body and from the surrounding 

 water. Little or nothing is known regarding its action in the protozoan 

 cell. 



Irritability. 'This liberation of energy is the 'response' to an 

 action of itself inadequate to produce it, and has been compared not 

 inaptly to the discharge of a cannon, where foot-tons of energy are 

 liberated in consequence of the pull of a few inch-grains on the trigger, 

 or to an indefinitely small push which makes electric contact; the 

 energy set free is that which was stored up in the charge. This capa- 

 city for liberating energy stored up within, in response to a relatively 

 small impulse from without, is termed 'irritability;' the external 

 impulse is termed the 'stimulus." (Hartog, 1906, p. 8.) The sensi- 

 tiveness or irritability of protozoan protoplasm has been a favorite 

 branch of protozoon research, and is especially interesting in the light 

 of comparative psychology, for here is the prototype of higher animal 

 consciousness. It is manifested in a great variety of ways, and the 

 manifestations have been grouped into categories called taxes or 

 tropisms. Nearly all of these reactions take the form of motion in 

 some form or other, and are usually called out in response to stimuli, 

 which may be of various kinds. Mechanical stimuli, light and heat 

 rays, electricity, diffusing chemical substances, all exert some effect 

 on the movements of protozoa, sometimes toward the source of stimu- 

 lation (positive taxis), sometimes away from it (negative taxis). It is 

 this irritability of protoplasm that frequently saves the life of the small 

 organism, or provides it with food. Positive thigmotaxis is the name 

 given to that reaction of a paramecium, for example, when it 

 approaches and adheres to some larger object where its bacterial food 

 may be concentrated; positive chemiotaxis is the reaction shown in 

 the sudden extension of the tentacles of actinobolus; positive or nega- 

 tive aerotaxis is that reaction whereby the organism so places itself 

 in a medium that irritability is reduced to a minimum, and so on, 

 all movement probably being a response to stimuli which owe their 

 origin either to external or internal causes, the latter due, perhaps, to 

 the varying conditions of hunger, fatigue, and the like. 



The most extensive and illuminating observations on this aspect of 

 protozoan physiological activity have been made by Jennings, and the 

 results of his long studies on the behavior of lower organisms are 

 well stated in his own words in the following theses (Jennings, 1906, 

 p. 261): 



1. "First, we find that in organisms consisting of but a single cell, 

 and having no nervous system, the behavior is regulated by all the 

 different classes of conditions which regulate the behavior of higher 

 animals. In other words, unicellular organisms react to all classes 

 of stimuli to which higher animals react. All classes of stimuli which 



