KINGDOM ANIMALIA Phylum Nematoda

Phylum Nematoda Bilaterally symmetrical Surrounded by a strong, flexible cuticle. “Pseudocoelomate” body plan: fluid-filled cavity contains an intestine and oviducts or testes. Simple nervous system consisting of dorsal and ventral nerve cords. Movement by contraction of longitudinal muscles against pressurized fluid. Hydrostatic skeleton. Some have specialized cells that excrete nitrogenous wastes; in others, canals or canals plus these specialized cells are present. No cilia or flagellae.

Reproduction Most nematodes are either male or female, not hermaphroditic. Fertilization takes place when males use special copulatory spines to open the females' reproductive tracts and inject sperm into them. The sperm are unique in that they lack flagellae and move by pseudopodia, like amoebas.

Nematode Ubiquity Nematodes are unbelievably abundant. One study reported around 90,000 individual nematodes in a single rotting apple. Up to 236 species have been found living in a few cubic centimeters of mud. The number of described species is around 12,000, but the actual number of species may be closer to 500,000.

Ecological Role Many nematodes are free living and play critical ecological roles as decomposers and predators on microorganisms. Some species are generalists, occurring across wide areas and in many habitats. Others are much more specialized. One species is known only from felt coasters placed under beer mugs in a few towns in Germany.

Parasitic Roundworms Some nematodes affect humans directly or indirectly through their domestic animals. Hookworms, live in small intestine of host Trichina, the worms that cause trichinosis Pinworms, transmitted from human to human by eggs floating in household dust Filarial worms, primarily tropical parasites that cause diseases such as filariasis (elephantiasis) by blocking lymphatic vessels