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(also Aschelminthes), a phylum of invertebrates including the classes Gastrotricha, Kinorhyncha, Rotatoria, Nematoda, Nematomorpha, Acanthocephala, and Priapulida. The unsegmented body is more or less completely covered with a cuticle. The sense organs include eyes, olfactory pits, and tactile organs. Between the dermomuscular tube and the gut there is a broad buccal cavity. The alimentary canal begins with a rudimentary mouth and ends in an anus. The excretory organs are protonephridia, or modified skin glands. There is no respiratory or circulatory system. The nerve system consists of a brain and nerve stems. The sexes are separate.
Some zoologists treat the phylum as a subphylum of Scolecida.