106 



THE ASCARIS 



Fig. 52. — Representative roundworms. A, the dagger nematode, Xiphinema, 

 an injurious form, which coils itself about the rootlets of plants in such a way 

 that the spearlike structure at the anterior end can be thrust far into the 

 tissue of the rootlet upon which th3 worm feeds. B, diagram showing by 

 stippling the relative abundance of nematodes of all kinds in each successive 

 two inches of a low-lying aUuvial soil estimated to contain three billion 

 nematodes per acre. Though shown distributed uniformly in each layer, the 

 worms are really most numerous about the roots of plants. C, carnivorous 

 nematode that feeds upon other nematodes. The three jawlike parts of the 

 mouth are armed with teeth, and the head has tentaclelike projections which 

 presumably are sensory. D, one of the Acanthocephala, the spiny-headed 

 worm, EchinorhyncJiUS dims, parasitic in the intestine of certain fishes, show- 

 ing the proboscis armed with hooks which serve for attachment to the host. 

 The internal structures, which consist almost wholly of reproductive organs, 



are shown in optical section. 



{A, B, and O, from N. A. Cobb, 1914, Yearbook U. S. Dept. Agriculture. D, from 

 H. J. Van Cleave, 1931, Transactiona American Microscopical Society, vol. 50. Drawn to 

 different scales.) 



