Greek: nema = filament, zoon = animal.
Phylum of Metazoa, group of Ecdyzoa.
The nematodes are elongate worms ranging in length from 0.3 mm up to the 8.5 m of Placentonema gigantissimum in the placenta of whales; they may inhabit soil, freshwater and saltwater habitats, and are frequently encountered as parasites of plants, humans, or animals. In general they are dioecius and in many species clear sexual dimorphism exists (Fig. 12, Hookworms/Fig. 1). Males are usually smaller than females (Table 1); both may have copulatory organs. The bilaterally symmetrical body of the unsegmented nematodes is covered by a typical cuticle which is formed by a hypodermis and must be shed during molt (Fig. 8, Ecdysis). The pseudocoelomatic body cavity of adults mostly contains a complete digestive tract, the anus of which is subterminally situated (Figs. 11, 12).