INTRODUCTION 



Any animal or any plant may be studied from several different 

 points of view, four of which are concerned in the present volume. 

 We may study its structure, ascertaining the parts of which it is 

 composed and the way in which these parts are related to each other. 

 This is the field of Anatomy. If we go into the more minute struc- 

 ture, for which the microscope has to be used, we are entering the 

 special anatomical field of Histology. When two or more different 

 animals are compared in points of structure, their resemblances and 

 differences being traced, the study is called Comparative Anatomy, 

 and it is only through such comparisons that we are able to arrive at 

 the true meanings of structure. Then it is of interest to see the way 

 in which the structure comes into existence in development from the 

 comparatively simple egg from which it arises — the province of Em- 

 bryology or Ontogeny. Anatomy and ontogeny together give us 

 a knowledge of the form and how it has arisen, and they are frequently 

 grouped as Morphology. But morphology merely deals with the 

 parts of a machine and these are usually studied in the dead organ- 

 ism; fully to appreciate the mechanism we should know how the 

 parts and the whole perform their work, the study of function or 

 Physiology. 



In view of the foregoing the present volume is to be regarded as 

 rather a comparative morphology of vertebrates, with here and there 

 hints at the physiological side. Farther, there is an adaptation of 

 the organism to the conditions in which it has to live, and the inter- 

 actions of this environment upon the animal have to be considered, 

 at least to a slight extent. 



Zoologists divide all animals into two great groups, the Protozoa, 

 in which the organism consists of a single cell, and the Metazoa, in 

 which the body is composed of many cells, which vary according to 

 the functions they have to perform. Of the Metazoa there are 

 several divisions — Porifera (sponges), Ccelenterata (sea anemones, 

 jelly fish), Echinoderma (starfish, sea urchins), Platodes (fiatworms), 

 Rotifera, Coelhelminthes (ordinary worms), MoUusca, Arthropoda 

 (crabs, insects), and Chordata. 



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