The bodies of nematodes(the Nemata)are round and long,tapering to a fine point at both ends.They lack cilia or flagella;even their sperm are devoid of flagella(they have an amoeboid structure).

　　Nematodes are encased in a tough cuticle.Beneath the cortex of the body wall lie bundles of longitudinal muscles,with no circular muscles.Between the intestine and the body wall is a fluid-filled body cavity(the pseudocoelom).

　　A distinctive feature of nematodes is their rounded shape,which results from the particularly high hydrostatic pressure of the fluid within the pseudocoelom.

　　In earthworms,the internal pressure is about 1.5–21 mmHg;in some nematodes,it can reach as high as 225 mmHg.The fluid-filled body cavity acts as a hydrostatic skeleton,and muscles move the body by pushing against this hydrostatic skeleton.

　　The combination of high internal pressure,a tough cuticle,and the absence of circular muscles means that nematodes cannot perform a wide range of movements.Consequently,they often move by thrashing or rolling—a characteristic feature of their locomotion.

　　Nematode populations are enormous,and they are found in nearly every habitat.Many live freely in soil or water,and most are so small that they require a microscope to be seen.

　　A single shovel of garden soil may contain one million or more nematodes.A rotten apple may harbor 90,000 nematodes.Many species are parasitic in plants and animals;they are usually very small,but some species can reach up to a meter in length.

　　Nematodes parasitizing crops cause millions of dollars in losses each year.Human infection with nematodes is also very common;statistics show that approximately 98%of the human population has been infected with nematodes at some point in their lives.