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 com- 

 posed 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 structure, 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 Embryology or Ontogeny. Anat- 

 omy and ontogeny together give us a knowledge of the form and how 

 it has arisen and they are frequently grouped as Morphology. But mor- 

 phology merely deals with the parts of a machine and these are usually 

 studied in the dead organism; 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 (flatworms), Rotifera, 

 Ccelhelminthes (ordinary worms), Mollusca, Arthropoda (crabs, 

 insects), and Chordata. 



