342 PRINCIPLES OF SOIL MICROBIOLOGY 



III. Trochelminthes or Trochalworms, represented in the soil by the Rotatoria 



or wheel animalcules. 



IV. Coelhelminthes (Annelida) or Segmented Worms are represented in the soil 



by the Oligochaeta, including the earthworms or Terricolae and the 

 Enchytraeids or Limicolae, and the Tardigrada. 

 V. Arthropoda are represented in the soil by (1) Crustaceae, especially 

 Copepoda and Isopoda; (2) Arachnida, including the mites, ticks and 

 spiders; (3) Myriapoda;. and (4) Insecta. 

 VI. Mollusca, including the Gastropoda. 



VII. Chordata. The vertebrates are represented in the soil by the mice, moles, 

 marmots, etc., but these are beyond our field of discussion. 



Methods of study. For the investigation of the soil fauna, Morris 1 

 devised an apparatus, which consists of four iron plates, two 12 by 10 

 inches, one 12 by 9 and one 4 by 9 inches. Each plate has an iron bar 



1 Morris, H. Observations on the insect fauna of permanent pasture in 

 Cheshire. Ann. App. Biol., 7: 141-155. 1921; On a method of separating in- 

 sects and other arthropods from soil. Bull. Entom. Res., 13: 197. 1922; The 

 insect and other invertebrate fauna of arable land at Rothamsted. Ann. App. 

 Biol., 9: 282-305. 1922. 



PLATE XVIII 

 Soil Nematodes 



144. The relative abundance of nematodes in each successive two inches of 

 upper foot of soil; derived from a low-lying alluvial soil containing about 

 3,000,000,000 nematodes to the acre, most of which are in the upper 3 inches, 

 around the plant roots (from Cobb). 



145. Beneficial soil nematode, Mononchus papillatus Bastion: it feeds on other 

 nematodes, showing remnants of several Tylenchuli (J, t) (from Cobb.) 



146. Assymetrical nematode Bunonema, found in decomposing organic matter 

 (from Cobb). 



147. Iota, or scaly nematode, common in the soil; head and tail end of male and 

 female (from Cobb). 



148. Male and female parasitic nematode, very simple in structure in com- 

 parison with free living nematodes (from Cobb). 



149. Tylenchus devastatrix infecting onions and other bulbous plants (from 

 Cobb). 



150. Mononchus attacking Anguillula aceti (from Steiner and Heinly). 



151. Sketch of the head-end of Mononchus attacking a larval Rhabditis (from 

 Steiner and Heinly). 



152. Schematic representation of the behavior of two different populations of 

 Tylenchus dipsaci. The one population lived on Hyacinths, the other on Nar- 

 cissus. Therefore, if left to choose, the first population will ignore the Narcissus, 

 the second the Hyacinth, for each will attack only the host of its ancestors. 

 (Slogterem, after Steiner). 



