zooLoor. 
an examination we shall obtain a fair idea of the form and 
structure of the great class of mammalia, or mammals, of 
which the dog and cat are examples. Moreover, we should 
study how the animal walks, how its heart beats, or its eyes 
see. This is studying the 2)liym)l()(jy of the animal. Then 
we should learn how the animal grows or develops from 
the^egg, and this is called Embryology^ the germ of an ani- 
mal being called an embryo. The bodies of animals are 
made up of cells. A cell is a microscoi)ic portion consist 
ing of a jelly-like substance called protoplasm. Animal- 
cules are comi)osed of but a single cell; such creatures are 
said to be unicellular, but most animals are formed of bone 
or shell, muscles, nerves, etc. These parts are made up of 
cells. Hence these animals are many-celled. The cells 
form tissues, such as muscular or nervous tissue. The 
study of cells and tissues is called Histology, Finally, we 
should acquaint ourselves with the habits and mental traits 
of the animal, and this is called Psychology, 
A fish is the most convenient vertebrate for ordinary 
school laboratory work. The object of these lessons is to 
induce the scholar to depend as far as possible upon the 
use of his own eyes and brains. He should observe with care 
some of the common animals here described, most of which 
he can readily obtain, and then study their form, habits, 
and the leading features of their anatomy. After examin- 
ing a starfish, clam, lobster, insect, and fish, and reading 
about their mode of growth, he will obtain a knowledge of 
the principal groups of the animal kingdom which he will 
remember throughout life. 
Classification. — There are estimated to be upwards of 
250,000 species of animals now living on the surface of 
the earth. How all these forms are related and how they 
differ comprises what is called the classification of animals 
or Systematic Zoology. 
When Linnaeus, the father of natural history, undertook 
to classify animals he divided the animal kingdom into 
classes, orders, genera, and species. Thus at present all 
