346 PRINCIPLES OF SOIL MICROBIOLOGY 



various habitats. They are distributed by the wind, by water, by 

 moving animals, by various plant products, implements, etc. The 

 eggs and larvae are sometimes very resistant to drying and other 

 adverse conditions, and can survive for many years. Large numbers 

 of parasitic, saprophytic, and free-living species inhabit the soil, making 

 up a large numerical proportion of its population. The number of 

 species alone reaches many thousands. Some of these are of wide 

 distribution. 



The identification of soil nematodes may be carried out by using 

 fixed material. Fleming's solution (p. 323) can be employed for this 

 purpose. The organisms are placed in the solution from a few minutes 

 to one or two hours. Should the cells become darkened, they can be 

 bleached with H 2 2 . When the nematodes are dead and fixed, they 

 are mounted, counted and identified, or are placed in a mixture of 

 5 per cent glycerol and 95 per cent water. After the water has evapo- 

 rated, the animals remain in the glycerol. For careful identification 

 and detailed study of morphology, the nematodes are placed in the 

 middle of a glass slide in a small drop of water and covered with a 

 cover glass. The edge of the cover glass can then be sealed to the slide 

 by means of hot wax — paraffin which contains a certain proportion of 

 beeswax. The slides are now examined with the compound micro- 

 scope, very high power lenses being necessary. deMan 7 (1884) was 

 the first to make a careful study of the soil nematodes. He divided 

 the organisms into three groups: (1) omnivagous species not bound 

 to any particular soil (Dorylaimus obtusicaudatus, Monohystera Jilifor- 

 mis, etc.); (2) meadow and field soil nematodes (Plectus cirratus, 

 etc.); (3) sand nematodes (Mononchus parvus, etc.). 



Nematodes are generally found to be abundant in forest humus as 

 well as in cultivated soils. They are parasitic on animals and plants, 

 or are saprophytic and free living. Even the parasites may lead an 

 independent existence in the soil at certain stages of their development. 

 According to Steiner, 8 the nematodes are represented in Swiss soils 

 by 139 known species, but there might be still many more. Cobb 9 

 found nematodes to occur in large numbers in every cultivated and 

 uncultivated soil, including forms which are parasitic on plants or 



7 de Man, 1922 (p. 348). 



8 Steiner, G. Freilebende Nematoden aus der Schweiz. Arch. Hydrobiol. 

 Planktonk., 9: 259 276. 1913; Zool. Anz., 46: 336-368. 1916. 



' Cobb, N. A. Nematodes and their relationships. U. S. Dept. Agr. Year- 

 book, 1914, 457-490. 



