18 ANIMAL BIOLOGY 



B. Unique Characteristics of Living Matter 



Since the phenomena of life are without exception the results of 

 protoplasmic activity, it is obvious that we must look to proto- 

 plasm for the primary attributes of living matter. The properties 

 which are absolutely characteristic of living matter are its spe- 

 cific organization, chemical composition, metabolism involving the 

 power of maintenance, growth, and reproduction, and irritability 

 resulting in the power of adaptation. 



1. Organization 



It must be emphasized that living things are not homogeneous, 

 but possess structural and physiological organization. Animals 

 and plants are made up of various parts adapted for certain pur- 

 poses. They exhibit 'a viable unity' and so stand in sharp con- 

 trast with objects comprising the inorganic world as, for instance, 

 rocks and rivers. Accordingly animals and plants are referred to 

 as organisms. Moreover, as we have seen, the organizational 

 units of all living things are cells, and so it follows that cell struc- 

 ture is a direct or indirect expression of all the unique life char- 

 acteristics that we are about to survey. A few of the details of 

 cell structure are necessary for an appreciation of the organization 

 of organisms. 



It will be recalled that the protoplasm of all typical cells is dif- 

 ferentiated into two chief parts: the cytoplasm, or general ground- 

 work which makes up the bulk of the cell; and the nucleus, a more 

 or less clearly defined spherical body, situated near the center of 

 the cytoplasmic mass. 



Cytoplasm. The cytoplasm may be considered the less special- 

 ized protoplasm of the cell, and its appearance and other charac- 

 teristics are those which have been outlined in our discussion of 

 protoplasm. With that in mind, for the sake of definiteness, we 

 may consider its basis as consisting of a meshwork, composed of 

 innumerable, minute granules which permeate an apparently homo- 

 geneous ground-substance, or hyaloplasm. Distributed through- 

 out the cytoplasm are usually various lifeless inclusions such as 

 granules of food, droplets of water or oil, vacuoles of cell sap, 

 crystals, etc., representing materials which are to be, or have been, 

 a part of the living complex, or are by-products of the vital proc- 

 esses. This passive material is frequently referred to as meta- 



