Systematic Position and Phylogeny

Nematodes are colloquially referred to as roundworms but this is rarely reflected in their gross appearance and refers mainly to their shape in cross section. The complete worms are often thread-like, cylindrical, generally fusiform, more rarely sac-like (e.g. female Tetrameres and Meloidogyne). They vary in length from microscopic (e.g. the females of Aphelenchoides bicaudatus which only reach up to 0.47 mm long) to several metres in length (e.g. the females of Placentonema gigantissima which have been recorded as over 8 metres long). The body is covered with a cuticle which may or may not exhibit any number of variations of complex ridges, spines or hooks. The body has no internal segmentation and beneath the cuticle the wall is composed of an epidermis (hypodermis) and a single layer of muscle cells.

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