INTRODUCTION 3 
which may enter the organism at any point, and consequently 
plants require no mouth or digestive cavity, or organs for the pre- 
hension of food. Animals, on the other hand, require more com- 
plex compounds; their nitrogen, with scarcely an exception, must 
be supplied in the form of proteids, and their carbon in the form of 
starch, sugar, or fat. Some of these compounds are not soluble, 
and hence an animal must ingest its food in a more or less solid 
state; and to that end it is usually provided with a mouth and 
digestive tract, with organs for the prehension of food, and with 
locomotor organs so that it may find its food. Since the food 
of animals does not exist in nature except as the products of 
living beings, it is obvious that animals are ultimately depend- 
ent on the plant world for their means of subsistence. 
The broken-down products of the protoplasm are 
usually excreted by special organs set apart for this purpose 
in animals, but in plants the waste products are either diffused 
from the surface of the organism, or are stored away in the 
plant. There are no special excretory organs. 
In both plants and animals the most lowly organised beings 
consist of one cell, and the unicellular organisms are termed 
the Protophyta and Protozoa respectively. The Metaphyta and 
Metazoa, or the multicellular plants and animals, consist of a 
number of cells arranged in more or less definite tissues, but 
even these multicellular beings pass through a unicellular stage, 
that of the ovum, whose repeated divisions after fertilisation 
give rise to the cells composing the body of the animal or 
plant. 
The Protozoa are therefore the simplest and most primitive 
animals, and it is natural to place them at the bottom of the 
animal kingdom. 
