PROTOPLASMIC FUNCTION 



37 



FIG. 10 



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EXCRETION. Excretion is the third of the essential processes involved 

 in nutrition. It is made necessary by the four circumstances that most 

 food contains an indigestible element; that the parts of an organism 

 select what they wish and reject the rest; that some digestible food 

 escapes digestion, for one reason or another; and, most important of all 

 theoretically, that the katabolism of 

 the tissues produces much material 

 not only lacking in any value to ani- 

 mals, but actually poisonous to them 

 if retained. Thus we see that the 

 removal of waste products is an indis- 

 pensable process, not only practically 

 but theoretically, for the products of 

 katabolism are largely either poisonous 

 in themselves or the natural food of 

 harmful bacteria which in turn produce 

 toxins that are often of a deadly nature. 



Irritability. Irritability is the term 

 which is used to designate all the 

 functions or vital manifestations of pro- 

 toplasm not included in nutrition. 



Irritability may be defined, perhaps 

 too tersely, as the reactibility of pro- 

 toplasm to stimulation, a definition 

 indicative of the general meaning of 

 the term. Indeed, as has already 

 been indicated, irritability is not con- 

 fined to animals or even to organisms, 

 for gunpowder, as Thompson says, 

 acts vigorously to the stimulation of a 

 spark, and similar examples are, of 

 course, numerous. Irritability is then 

 the most general property of proto- 

 plasm. If respiration, nutrition, and 

 reproduction are discussed under dif- 

 ferent heads, it is for convenience and 

 because of their separate importance, 

 rather than because these phenomena 

 are outside the manifestations of irri- 

 tability. Thus, respiration is a sys- 

 tematized reaction to the stimulation 

 of an excess of carbon dioxide; nutrition, a reaction to a lack of oxygen 

 and of food, and reproduction, a reaction to complex sexual strains 

 and processes. 



MOVEMENT. Movement is the most conspicuous of the animal 

 reactions to stimulation. If a speck in a distant landscape or in the 

 field of a microscope moves, the presumption is almost instinctive that 



Grovia oviformis feeding. (Dubois.) 



