The Scope of Zoology 



Students beginning a study of zoology often have vague and some- 

 times false impressions about the nature of this subject. It will be well, 

 therefore, for us to devote the first chapter to a consideration of the 

 scope of the subject of zoology so that those of you who are embarking 

 upon your first adventure in this field of science will have some con- 

 cept of what to expect along the way. Zoology is a study of those forms 

 of life which come under the broad heading of "animals." Many of you, 

 no doubt, have come to think of animals in a somewhat narrow sense — 

 you may think that the term is restricted to the fur-bearing animals 

 which have their young born alive. With this viewpoint you would 

 not think of a bird, a fish, or a grasshopper as an animal. In the 

 broader sense, however, animals include all of those forms of life which 

 are not plants (excepting a few microscopic forms which do not fall 

 readily into either classification). This is the sense in which the word 

 is used in its proper form. With such a broadened viewpoint you can 

 readily see that worms, snails, jellyfish, and human beings would all be 

 animals just the same as are cats, dogs, monkeys, cows, and hyenas. 



The number of different kinds of animals which inhabit the earth 

 is very great. Something like a million different species of animals have 

 been studied, named, and described. Among these there is a great va- 

 riety — some animals are so small in size that they cannot be seen without 

 the aid of the microscope, whereas at the other extreme are animals 

 whose weight is measured in tons. Some of the smaller forms of ani- 

 mals have bodies that consist of single cells, while some of the larger 

 forms have literally billions of cells that are organized into complex 

 mechanisms of tissues and organs. 



Since it is usually easier to proceed from the simple to the complex 

 in any field of study, we will take up the various forms of animal life 

 somewhat in the order of increasing complexity. The first animals we 

 will survey will be the one-celled forms in which all the vital life proc- 

 esses are concentrated into a single cell. By learning about the nature 

 and activities of these one-celled forms, you will be learning about life ; 

 and the more you learn about life in any of its manifestations, the more 

 you will know about all forms of life. A knowledge of the functions of 



1 



