218 



FIRST YEAR SCIENCE 



vertebrates (with backbone). The invertebrate is the 

 much more numerous class as it contains the worms, shell- 

 fish, insects and those almost countless forms of animal 

 life which have no internal bony skeleton and backbone. 

 The higher animals, like fishes, amphibia, reptiles, birds and 

 mammals, belong to the class of vertebrates. Man himself 

 is the highest of the vertebrates, and as the purpose of 

 this book is to study the earth and its relation to man, his 

 structure will be studied later. 



102. Invertebrates ; Protozoa. The very lowest form of 

 animal life, the protozoa, are single-celled animals. In some 

 species they are very difficult to distinguish from plants. 

 They are microscopic in size and most of them live in 

 water. Our chief interest in them in the present study is 

 that they are the cause of several kinds of disease which 

 can readily be prevented with proper care. Malaria, and 

 the terrible African disease called the sleeping sickness, 

 and probably yellow fever are due to these little animals. 

 Unlike bacteria, the protozoa do not cause disease by 

 passing directly from one person to another, instead they 



need to live in some 



insect between whiles. 

 In malaria and yellow 

 fever the insect in 

 which they live is the 

 mosquito, and in the 

 sleeping sickness they 

 live in a fly called the 

 tsetse. If a mosquito 

 of the right species bites 



A DISEASE-BEARING MOSQUITO. m- , ^ -,1 



a person afflicted with 



Greatly magnified. 



malaria or yellow fever, 



some of these little animals, the protozoa, are sucked up with 

 the blood and enter the mosquito. They grow in its body, 



