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Priapulida,phylum consisting of 17 species of predatory, unsegmented marine worms that live in the sand and mud at the sea bottom. The largest are 4 to 6 in. (10–17 cm) long, but the majority of species are less than .05 in. (.13 cm) in size. The animals consist of a spiny body with an anterior that can be everted or inverted into the trunk for locomotion or feeding. Spines around the mouth and in the pharnyx are everted to capture prey. The sexes are separate and the egg is fertilized externally, hatching into a larval stage in all but one species. Fossil species can be traced to the Cambrian period, but zoologists do not agree on an interpretation of the lining of the body cavity. This would determine if priapulids are coelomates or pseudocoelomates.
One of the minor groups of wormlike marine animals, now regarded as a separate phylum of the animal kingdom with uncertain zoological affinities. The phylum is a small one with only two genera, Priapulus and Halicryptus.
Priapulida inhabit the colder waters of both hemispheres. They burrow in mud and sand of the sea floor, from the intertidal region to depths of 14,850 ft (4500 m).
Priapulids are small to medium-sized animals, the largest specimen attaining 6 in. (15 cm) in length. The body of Priapulus is made up of three distinct portions: proboscis, trunk, and caudal appendage (see illustration). Separated by a constriction from the trunk, the bulbous, introversible proboscis usually constitutes the anterior third of the body and is marked by 25 longitudinal ridges of papillae or spines. The mouth is located at the anterior end of the proboscis and is surrounded by concentric rows of teeth. The cylindrical trunk is annulated, but not segmented, and is often covered with irregularly dispersed spines and tubercles. At the posterior end of the trunk there are three openings: the anus and two urogenital apertures.
a small relict group of marine worms of indeterminate taxonomic position. The body ranges in length from a few millimeters to 20 cm. The anterior third of the body, the proboscis, can be drawn inside by means of special muscles (retractors) and can extrude by pressure of cavitary fluid. The presence of a coelom links priapulids to the higher worms; however, unlike most higher worms, priapulids have an unsegmented body. The circulatory system is absent. Respiration occurs through the surface of the body or by means of posterior gill appendages. The nervous system consists of an esophageal ring and a ventral cord, and the excretory and sexual organs are united in a urogenital system. The sexes are separate. Some zoologists believe that the worms are closely related to Nemathelminthes owing to their type of egg cleavage.
There are 8 species of priapulids, embracing 6 genera. In the seas of the USSR, usually two genera, Priapulus and Halicryptus, are encountered.