ZOO LO G Y. 
INTRODUCTION. 
Definition of the Subject.— The science that 
treats of organic nature, or living things, is termed Zv- 
ology. It is divided into Botany, that treats of plants; 
and Zodlogy, that relates to animals, 
The Cell.—All animals are made up of one or more 
cells, minute globules of a jelly-like substance called 
protoplasm, as a rule inclosed in a delicate covering or 
membrane. The protoplasm of genuine cells generally 
contains minute moving granules, and a round transpar- 
ent body termed the nucleus, that contains a dark ob- 
ject, called the nucleolus. The lowest animals are single 
cells, and are termed unicellular, but in the higher forms 
the cells separate by self-division, and form two layers: 
outer (ectoderm), inner (endoderm), while a middle layer 
is called mesoderm. From these cell-layers bone and 
muscular tissue are formed, and the animals are said to 
be many-celled. 
Difference between Animals and Plants.—It is 
not difficult to distinguish between the higher forms of 
animals and plants. The trees are sightless, have no 
locomotive organs, and, as a rule, live upon inorganic 
substances ; yet there are curious points of resemblance. 
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