36 NORTH AMERICAN 



costing civilized nations hundreds of thousands of dollars yearly 

 for the inspection of pork, is a nematode. If trichina,-infested or 

 measly pork be eaten by human beings, the result is a serious, often- 

 times fatal, sickness, called trichinosis, epidemics of which have 

 claimed victims by the hundred. Nematodes have recently been 

 suspected, with good show of reason, of being carriers of cancer. 

 So the list of serious human nematode diseases might be increased 

 until practically half a hundred had been enumerated. 



No less serious are the nematode diseases of plants and of 

 the lower animals. The common gall-worm has been found infest- 

 ing the roots of several hundred different species of plants, among 

 them most of our cultivated crops, and causes a'n annual loss 

 amounting to millions of dollars. There is another nematode that 

 has at times completely checked the growing of sugar beets in cer- 

 tain regions. The list of serious plant diseases of this character 

 could easily be increased to scores. The same is true of animals. 

 Every domestic, doubtless every wild, species has a number of 

 specific nematode parasites sapping its vitality. 



WONDERFUL VARIETY OF HABITAT. 



Not the least interesting thing about nematodes is the astound- 

 ing variety of their habitats. They occur in arid deserts and at the 

 bottoms of lakes and rivers, in the waters of hot springs and in 

 polar seas where the temperature is constantly below the freezing 

 point of pure water. They were thawed out alive from Antarctic 

 ice in the far south by members of the Shackelton expedition. They 

 occur at enormous depths in Alpine lakes and in the ocean. As 

 parasites of fishes they traverse the seas ; as parasites of birds they 

 float across continents and over high mountain ranges. Their eggs 

 and larvae, invariably of microscopic size, are carried from place to 

 place by an exceedingly great variety of agencies. Almost any vis- 

 ible thing that moves is capable of transporting nematode eggs or 

 larvae. Sometimes the eggs and larvae are so resistant to dryness 

 that if converted to dust they revive when moistened. This 

 revival of mummified nematodes may take place after as long a 

 period as a quarter of a century. 



Nematodes are found in queer places. The wildest imagina- 

 tion could hardly outpicture the facts. One species is found prac- 



