Priapulans

Phylum Priapulida Number of families 3

Thumbnail description

Cylindrical body, the anterior part of which—the introvert—is covered with chitinous teeth and can be rolled inward; at the posterior end are one or two caudal appendages

Illustration: Tubilucus corallicola. (Illustration by John Megahan)

Phylum Priapulida Number of families 3

Thumbnail description

Cylindrical body, the anterior part of which—the introvert—is covered with chitinous teeth and can be rolled inward; at the posterior end are one or two caudal appendages

Illustration: Tubilucus corallicola. (Illustration by John Megahan)

Loriciferans

Evolution and systematics

Priapulans once were grouped among a diverse collection of invertebrates known as the Aschelminthes. Modern comparative studies place the priapulans with kinorhynchs and loriciferans in a group called Cephalorhyncha or Scali-dophora. These three phyla have chitinous cuticles and rings of scalids on the introvert. Sixteen living and approximately 14 fossil priapulans are known. The living species are generally classified into three families.

Physical characteristics

The body of a priapulan is divided into three parts—introvert, trunk, and caudal appendage. The introvert can be pulled completely into the trunk by a pair of retractor muscles. Chitinous scalids of various sizes and shapes cover the entire surface of the introvert. Sometimes the scalids at the anterior end of the introvert are larger than those nearer the trunk. Within the introvert is a muscular pharynx armed with cuticular teeth. The trunk houses the internal body organs, in particular the digestive system and reproductive organs. The body of priapulans is filled with fluid that acts a hydrostatic skeleton as the body wall muscles contract. During movement, the fluid moves around in the body cavity and serves the functions of circulation, excretion, and respiration. The caudal appendage is continuous with the body cavity of the trunk. The function of the caudal appendage has not been established; it may serve a respiratory function.

Distribution

All oceans from shallow water to the deepest parts of the sea. In some areas only one or two records exist, most likely because of infrequent collecting. Larger priapulans have been found in colder ocean waters, especially in the Northern Hemisphere. Small and interstitial priapulans are most common in the shallow tropics.

Habitat

Priapulans inhabit soft sediments of all kinds but are uncommon in areas with rocks. Larger species, such as Priapu-lus caudatus, are generally found in very soupy muddy bottoms. Some of the smaller species live in sandier sediments, where there is considerable interstitial space for movement. In the tropics, priapulans commonly are found in poorly sorted coral sand.

Behavior

Having no legs or other appendages, priapulans depend on the hydrostatic skeleton for movement. When extended, the introvert acts as an anchor in the sediment, as does the anterior part of the trunk when the circular body wall muscles of the central trunk are contracted. Once anchored, the priapu-lan can pull itself through the sediment by contraction of the longitudinal body wall muscles. Peristaltic contractions of the body wall muscles move the body through the sediment.

Feeding ecology and diet

The food source of the larger priapulans has not been ascertained. These animals apparently are capable of capturing and ingesting larger, slow-moving polychaetes. With the in trovert fully extended, priapulans can grasp prey with the teeth of the pharynx and rapidly roll it inward. In aquaria P. caudatus has been seen eating a variety of marine worms. Smaller individuals, however, may be mud eaters, as are smaller species. Examination of stomach contents has proved inconclusive about priapulan feeding habits.

Reproductive biology

The sexes are separate in all priapulans that have been studied. Gonadal products are released freely into the water, where fertilization occurs. In larger species, a loricate larva forms and lives in the bottom mud. As the larva grows, it sheds the cuticular covering and gradually grows into a juve nile priapulan. In at least one meiofaunal priapulan, the female broods the embryos, which hatch as juveniles.

Conservation status

Priapulans are not protected in any region, although they are designated as animals of special significance in Maine in the United States. No species is listed by the IUCN.

Significance to humans

This phylum was apparently very abundant in the Cambrian period. Species alive today are in some ways living fossils and should be conserved.

Hidradenitis Illustration
1. Priapulus caudatus; 2. Tubiluchus corallicola. (Illustration by John Megahan)
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