Seal 



Walrus 



TEETH OF FLESH-EATERS 



The large canine, or "dog", teeth act as weapons in fighting or grasping. The short 

 incisors cut tough tissues, and the heavy molars break and crush bones 



Takers and Sharers Nearly every species of plant and animal acts as 

 an unwilling "host" to one or more life forms that live at its expense. Com- 

 mon examples of parasites, as such uninvited guests are called, are the leech, 

 the sheep tick, the liver-fluke and the bedbug. Many diseases result from 

 the destructive action of parasites, such as the malaria plasmodium, the 

 Treponema pallidum, or syphilis parasite, the hookworm, and the bacteria 

 of many common diseases. 



An interesting partnership between two species is seen in the symbiosis 

 or "living together", of a species of termite and certain protozoa that live 

 within its digestive tract (see illustration, p. 179). The termite lives in dead 

 wood, in the forest or in buildings, mining through it by chewing the wood 

 into small bits, which it swallows. Within the digestive tract live the pro- 

 tozoa which produce enzymes that change the cellulose into soluble sugars. 



Periodic Feast and Famine All animals convert some of their surplus 

 food into fat. This is stored within the body and is used in times of emer- 

 gency or of food shortage. Some species, the bear and the woodchuck, for 

 example, feed and fatten during the summer months and spend the cold 

 months in a deep sleep, called hibernation, or "wintering". At this time 

 they live on the food stored during the summer feasting. 



Another illustration of getting food while the getting is good is seen in 

 the distinct stages characteristic of many species of insects (see illustration, 



177 



