50 BEGINNERS' ZOOLOGY 



body wall. As the segments at the older end mature, 

 each becomes full of eggs, and the segments become 

 detached and pass out of the canal, to be dropped and 

 perhaps picked up by an herbivorous animal and the 

 life cycle is repeated. 



The trichina is more dangerous to human life than is the 

 tapeworm. It gets into the food canal in uncooked pork 

 (bologna sausage, for example), multiplies there, migrates 

 into the muscles, causing great pain, and encysts there, 

 remaining until the death of the host. It is believed to 

 get into the bodies of hogs again when they eat rats, which 

 in turn have obtained the cysts from carcasses. 



Summary of the Biological Process. — An earthworm is 

 a living machine which docs work (digging and crawling; 

 seizing, swallowing, and digesting food; pumping blood; 

 growing and reproducing). To do the work it must have 

 a continual supply of energy. The energy for its work is 

 set free by the protoplasm (in its microscopic cells) under- 

 going a destructive chemical change (oxidation). The 

 waste products from the breaking down of the protoplasm 

 must be continually removed (excretion). The broken- 

 down protoplasm must be continually replaced if life is to 

 continue (the income must exceed the outgo if the animal 

 is still growing). The microscopic cells construct more 

 protoplasm out of food and oxygen (assimilation) supplied 

 them by the processes of nutrition (eating, digesting, 

 breathing, circulating). This protoplasm in turn oxidizes 

 and releases more energy to do work, and thus the cycle 

 of life proceeds. 



