INTRODUCTION. 
Any animal or any plant may be studied from several different 
points of view, four of which are concerned/the(it) 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 thestructure 
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), Coelenterata (sea anemones, jelly fish), 
Echinoderma (starfish, sea urchins), Platodes (flatworms), Rotifera, 
Ccoelhelminthes (ordinary worms), Mollusca, Arthropoda (crabs, 
insects), and Chordata. 
