INFUSORIA / 



cause response or reaction in living protoplasm are called 

 stimuli. The principal stimuli' may be classed as chemical 

 stimuli, differences in temperature, light, contact, electricity, 

 and gravity. Protozoans possess; (1) Irritability, that property 

 of living protoplasm which gi^•es it power to respond to stimuli; 

 (2) automatism, the power of movement, or of changing the form. 



Locomotion. — Protozoans move by means of pseudopodia, 

 cilia, or flagella. Some forms, as the Vorticel'la, are fixed, and 

 can move only by the contractility of their stalks or stems. 



Nutrition. — The food of protozoans is composed of whatever 

 minute organisms or fragments of organic matter they are 

 able to obtain in the water. The parasitic forms, of course, 

 simply absorb nutriment from the liquids of the host. The proc- 

 ess of nutrition in the simplest protozoan consists in wrapping 

 or, more correctly, flov/ing itself about the particle of -food, 

 absorbing the nutriment needed, and rejecting what it cannot 

 use. Thus we see that it has the power of selective absorption, 

 or digestion. 



Circulation is brought about by simply changing the form of 

 the botly mass, thus changing the position of the absorbed 

 nutriment in the one-celled body. 



Assimilation, or the making of this absorbed material into 

 its own body substance, next takes place, and, as a consequence, 

 growth. The using up of assimilated material for heat or 

 motion (energy), or metabolism, also takes place. 



Respiration, or the taking in of oxygen and the giving off 

 of carbonic acid gas and other wastes, is effected by the absorp- 



1 The reactions (orientation) of animals in response to these various 

 stimuli are called tropisms; the response to chemical stimuli is called 

 chemolropism ; to heat, thermotropism; to light, photoiropism ; to contact, 

 ihigmotropism ; to electricity, eledrotropism ; to gravity, geotropism, and so 

 on. Loeb and others claim that the movements of the lower forms and 

 many of those of the higher forms are purely physical and chemical reac- 

 tions, just exactly as those known to us in the inorganic world. H. S. 

 Jennings, who is another very careful investigator, asserts that his inves- 

 tigations show " that in these creatures their behavior is not, as a rule, on 

 the tropism plan — a set, forced method of reacting to each particular agent 

 — but takes place in a much more flexible, less directly, machine-like way 

 by the method of trial and error. . . This method loads upward, offer- 

 ing at every point opportunity for development, and showing even in the 

 unicellular organisms what must be considered the beginnings of intelli- 

 gence and of many other quahties found in higher animals." 



