INTRODUCTION 7 



crawl about on the bottom; wheel animalcules (Fig. 122) and 

 many other extremely small animals swim about in search of 

 food ; and almost every drop of pond water contains a number 

 of microscopic forms. 



The terrestrial animals are the ones best known to the average 

 person, and every one is aware of the vast numbers of deer, 

 wolves, field-mice, snakes, insects, and other forms that move 

 about on the surface of the earth. Animals like the mole and the 

 earthworm which live underground are said to be subterrestrial, 

 and those like the birds and butterflies that frequent the air are 

 called aerial. 



Parasites are more widely spread than is generally known. 

 Almost every animal is infested with others which prey upon it. 

 The malarial fever germ is one of the most important, although 

 one of the smallest, parasites. The fleas and lice are called 

 external parasites. The internal parasites of man include the 

 roundworm Ascaris (Fig. in), the tapeworm (Fig. 107), and 

 the Trichinella (Fig. 113). Frequently parasites are preyed upon 

 by other parasites, a condition known as hyperparasitism - 

 and even the hyperparasites may be parasitized. Thus the fol- 

 lowing humorous lines contain a grain of truth: - 



"Great fleas have little fleas 

 Upon their backs to bite 'em, 

 And little fleas have lesser fleas, 

 And so ad infinititm." 



The survey of the animal kingdom just concluded attempts to 

 present a few facts about the groups of animals to be studied in 

 the succeeding chapters. The most highly organized and most 

 familiar animals, the mammals, were considered first, and the 

 less complex were successively discussed in a descending series, 

 until the last and simplest organisms were reached. A glance at 

 the table of contents of this book will show that the extended 

 studies of these groups have been arranged in a reversed order, 

 beginning with the simplest animals, the PROTOZOA, and ending 

 with the highest type, the Mammal. This method of presenting 



