CHAPTER II 
THE ANIMAL ORGANISM AND ITS ENVIRONMENTAL RELATIONS 
I. NATURE OF LIVING SUBSTANCE 
The bodies of living plants and animals are made up of living matter 
known as protoplasm (35, chap. ii). Protoplasm is a chemical substance 
or a mixture of chemical substances. It is very difficult to distinguish 
living and non-living matter by definition. However, we experience 
little difficulty in separating living from non-living things. This is 
because living things usually possess certain definite forms and ability to 
reproduce and move (especially animals). They also possess irritability. 
This is the property by virtue of which the force applied to living: sub- 
stance is not in proportion to the force resulting (35, p. 124). One 
strikes a horse with a whip; the energy which the horse exerts in running 
is not proportional to the force of the blow, but is far greater. 
In considering the environmental relations of animals, we shall 
separate our discussion into that concerned with form and that con- 
cerned with movement (motor activity) and other functional manifesta- 
tions. The term function is understood to cover all action on the part 
of the various parts of the organisms, motor activity included (35a, 
chaps. vii, viii, and ix). 
Il. THE RELATION OF FORM OR STRUCTURE TO FUNCTION 
The term animal calls forth a mental picture of activity and 
movement. The animals with which we are most familiar are those of 
large size, such as fishes, birds, and mammals. They and the groups 
to which they belong represent only a very small part of the animal 
kingdom, but we may consider one of these familiar animals as an 
example of animals in general. The black bass will serve our purpose. 
Such a fish is a complicated, highly organized animal (36, p. 183), 
possessing many organs, such as fins, gills, teeth, a stomach, an intestine, 
a liver, a heart, and a brain and spinal cord harnessed to the rest of the 
body by a series of small nerves which control all the organs. The fins, 
which are the external organs of locomotion, are sufficient in number to 
control the body and force it forward. The muscles which move the 
fins must receive nourishment in order to do their work. The nourish- 
ment is carried in blood-vessels, and the fluid which bears the nourish- 
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