424 The Animal Kingdom 



of the earth; and as shipped from point to point throughout the civi- 

 lized world in vehicles of traffic." * 



From this very brief statement of the comparative abundance of 

 nematodes, it must be clear that their economic importance is very 

 great. The vast number of free-living species in the soil play a part 

 with the other microfauna in stirring up and aerating the soil. The 

 exact role played by the nematodes in maintaining soil fertility has 

 never been adequately studied, but undoubtedly their dead bodies con- 

 tribute materially. 



The nematodes that attack plants are often a serious menace. In 

 the artificial environment of greenhouses, soil nematodes attacking 

 roots often become so abundant as seriously to disrupt the process of 

 plant raising. In Central America, the root nematodes which attack 

 the banana plants have seriously affected the yield in some areas. An- 

 other species in Hawaii appears to be harming the pineapple. 



Nearly all animals are attacked by some species of nematode. 

 Some of these do considerable damage to the host, resulting in a 

 decline of vigor and even in death. Among human parasites, hook- 

 worm, pinworm, trichina worms, filarial worms, and ascarids are but 

 a few of the better known forms. Domestic animals likewise have 

 their own rich fauna of nematodes. 



Characteristics of the Phylum. — The exact relationships of the 

 members of this phylum have long confused zoologists ; however, it 

 appears that they arose from some primitive flatwormlike ancestor. 

 Like the flatworms, these worms have bilateral symmetry, cephalization, 

 three germ layers (triploblastic), and the organ grade of construction. 

 Unlike the acoelomate flatworms, however, the roundworms retain the 

 embryonic blastocoel as a permanent body cavity. Since this cavity 

 is not lined by peritoneum, it is known as a psciidocoel, in contrast 

 to the peritoneum-lined coelom of higher invertebrates and vertebrates. 



The noncellular cuticle forms the outermost layer of the body 

 wall of the roundworms. This cuticle is usually much heavier in 

 the parasitic than in the free-living forms, and probably gives protec- 

 tion against the host's enzymes. In many species, there are striations, 

 bristles, and at times spines ; in a few the striations give a nearly an- 

 nulate appearance to the body. Beneath the cuticle is the cellular 

 epidermal syncytium or hypodermis which projects into the pseudocoel in 



*Cobb: Nematodes and Their Relationships, Yearbook of the U.S. Depart- 

 ment of Agriculture. 1914, 



