INTRODUCTION 



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 beiags 

 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. 



