Viruses and Phages 



93 



Other Animal Forms 



Animal forms larger than protozoa also occur abundantly in the 

 soil. They range from microscopic nematodes to large earthworms 

 and insect larvae. Some nematodes 

 (Hetcrodera schachtii) and certain in- 

 sects are parasitic on plants; some 

 (hookworm larvae) are parasitic on 

 animals; others, such as nematodes that 

 attack Japanese beetle larvae, parasi- 

 tize plant parasites and are thus bene- 

 ficial. Many are saprophytic; these 



comprise the earthworms, which macerate the soil, mix the organic 

 with the inorganic contents, and thus greatly improve soil fertility. 



Fig. 



46. Parasitic nematode 

 (from Cobb). 



Viruses and Phages 



Certain viruses and various phages exist independently in the soil. 

 The mosaic \'irus of wheat can be transmitted from the soil. Heating 

 the soil for 10 minutes inactivates this virus. The survival in the soil 



Fig. 47. Saprophytic nematode attacking parasitic form (from Cobb' 



of phages active upon legume bacteria may become an important 

 economic problem in successful legume inoculation; the selection of 

 phage-resistant strains of bacteria may be the answer. Various ac- 

 tinophages have also been demonstrated in the soil. 



