This section of the Handbook of Zoology is intended as a comprehensive and exhaustive account of the biology of the taxa Gastrotricha, Nematoda, Nematomorpha, Priapulida, Kinorhyncha, Loricifera, Gnathostomulida, Micrognathozoa, Rotifera, Seisonida and Acanthocephala, covering all relevant topics such as morphology, ecology, phylogeny and diversity. The groups covered include the diverse Nematoda with ca. 20.000 described species. As with the first edition, the Handbook is intended to serve as a reliable resource for decades.