472 THE ANIMALS OF THE EARTH 



but move about by means of a strong muscle which they extend 

 beyond the shell. 



Oysters, clams, and all soft-bodied animals which are protected 

 by shells are called mollusks ; in some mollusks, such as snails 



(Fig. 342), there is only one shell; in other 



mollusks, such as oysters and clams, there are 



two shells. 



Flies. Four fifths of all animals are insects, 



a group which, except for the honeybee and the 

 FIG. 342. A snail, silkworm moth, do not contribute food and 



clothing to man. But this enormous group of 

 animals plays an important part in our lives, because some of 

 them, like the fly and the mosquito, spread disease ; others, like 

 the corn worm and the potato beetle, destroy crops; and still 

 others, like the roach, infest our dwellings. 



The two insects which are most injurious to health are the fly 

 and the mosquito. The fly seems harmless because of its size. 

 If it were so large that we could see the hairs which cover its 

 body and its six legs, we would not tolerate it for an instant, 

 because these hairs are always clogged with filth and germs. 

 On the end of each of the six feet is a tiny pad moistened by 

 a sticky secretion which enables the fly to walk on smooth sur- 

 faces such as window-panes, ceilings, and edges of dishes. As 

 the fly crawls from place to place the pads on the feet and the 

 hairs all over the body pick up dust and filth, and when the fly 

 alights on our meat, preserves, and bread, it contaminates the 

 food with filth. Flies are undiscriminating creatures, and 

 visit sputum in the street, sewage plants, and rotting dump 

 heaps as eagerly as they visit the food in our kitchens. The 

 filth gathered from street, dump heap, and sewage reeks with 

 disease germs, and food over which flies have walked is prob- 

 ably contaminated with the germs of tuberculosis, typhoid fever, 

 and other diseases, and is capable of causing illness. 



