This volume includes a general introduction, a chapter on fossil representatives of the Cycloneuralia and chapters on the taxa Nematomorpha, Priapulida, Kinorhyncha and Loricifera. The taxa described herein include the parasitic horsehair worms (Nematomorpha) and the marine priapulids, kinorhynchs and loriciferans, the latter of which belong to the smallest metazoan animals and have fascinating life cycles and ecological capabilities. The volume presents a detailed insight into the morphology, ecology and systematics of each of these groups.