SHIRLEY,T.C.: Priapulida

Priapulida are a monophyletic group of marine, benthic worms with a retractile introvert for locomotion and feeding and a chitinous cuticle which is shed periodically. A mixture of bilateral and radial symmetry is present. The integument may bear many unique structures, including scalids, flosculi, setae, hooks, trunk scalids, tubercles and tumuli. A spacious body cavity serves as a hydrostatic skeleton and contains a complex alimentary canal with a thick, muscular pharynx. Several blood cell types occur. Unusual protonephridia serve as excretory organs. Priapulids are dioecious, with internal or external fertilization; hermaphroditism and parthenogenesis may occur. Indirect development of demersal larvae is normal, but direct development occurs in one species. Morphology of larvae is less variable than in adults and three types of larvae are known. Distribution of 18 extant species is from the shallow arctic, to the deep ocean, to interstitial waters of tropical, sandy beaches. Sizes vary from meiofaunal (<0.5 mm) to macrofaunal (40 cm). Some extant species are morphologically similar to mid-Cambrian fossils. The feeding ecology of most species is unknown, but several macrofaunal priapulids are predaceous and help structure benthic communities; some are common prey items for marine mammals and fish. Some species have high tolerance of hypoxic conditions and hydrogen sulfide. Priapulida have close affinities with Kinorhyncha, Loricifera and perhaps Nematomorpha.