Priapulida

(invertebrate zoology) A minor phylum of wormlike marine animals; the body is made up of three distinct portions (proboscis, trunk, and caudal appendage) and is often covered with spines and tubercles, and the mouth is surrounded by concentric rows of teeth.

(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

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 Scalidophora. 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 un-common in areas with rocks. Larger species, such as Priapulus 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 priapulan 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 introvert 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 juvenile 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.

Species accounts

Resources

Kirsteuer, E., and J. van der Land. "Some Notes on Tubiluchus corallicola (Priapulida) from Barbados, West Indies." Marine Biology 7 (1970): 230–8. van der Land, J. "Systematics, Zoogeography, and Ecology of the Priapulida." Zoologische Verhandelingen 112 (1970): 1–118.

[Article by: Les Watling, PhD]

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.

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Priapulida. (a) Priapulus adult and (b) larva.

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (December 2007)

Priapulida
Illustration of Ottoia, a priapulida, burrowing. Haplophrentis is nearby.
Scientific classification
Kingdom: Animalia Phylum: Priapulida Théel, 1906

Priapulida (priapulid worms or penis worms, from Gr. πριάπος, priāpos 'Priapus' + Lat. -ul-, diminutive) are a phylum of marine worms with an extensible spiny proboscis. Priapulid fossils are known at least as far back as the Middle Cambrian. Their nearest relatives are probably Kinorhyncha and Loricifera with which they constitute the taxon Scalidophora. Besides arthropods and velvet worms, it is only among Priapulida that we can find members of the Ecdysozoa which are relatively large in size. They were likely major predators of the Cambrian period. There are 16 known species of Priapulid worms.

They are cylindrical worm-like animals, with a median anterior mouth quite devoid of any armature or tentacles. The body is ringed, and often has circles of spines, which are continued into the slightly protrusible pharynx. The alimentary canal is straight, the anus terminal, though in Priapulus one or two hollow ventral diverticula of the body-wall stretch out behind it. The nervous system, composed of a ring and a ventral cord, retains its primitive connection with the ectoderm.

There are no specialized sense organs or vascular or respiratory systems (hemerythrin is the protein responsible for oxygen transportation). There is a wide body-cavity, but as this has no connection with the renal or reproductive organs it cannot be regarded as a coelom, but probably is a blood-space or hemocoel.

The Priapuloidea are hermaphroditic, and their male and female organs, which are one with the excretory organs, consist of a pair of branching tufts, each of which opens to the exterior on one side of the anus.

The tips of these tufts enclose a flame-cell similar to those found in flatworms and other animals, and these probably function as excretory organs. As the animals become adult, diverticula arise on the tubes of these organs, which develop either spermatozoa or ova. These pass out through the ducts. Nothing is known of the development. There are three genera: (i.) Priapulus, with the species P. caudatus Lam. of the Arctic and Antarctic and neighboring cold seas, and P. bicaudatus, Dan., of the north Atlantic and Arctic seas; (ii.) Priapuloides australis, de Guerne, of the southern circumpolar waters; and (iii.) Halicryptus, with the species H. spinulosus, v. Sieb., of northern seas. They live in the mud, which they eat, in comparatively shallow waters up to 50 fathoms (90 m).

Classification

Phylum Priapulida

Genus †Ancalagon

Genus †Anningvermis

Genus †Corynetis

Genus †Ottoia

Class Priapulimorpha

Order Priapulimorphida

Family Priapulidae

Genus Acanthopriapulus

Genus Priapulopsis

Genus Priapulus

Family Tubiluchidae

Genus Meiopriapulus

Genus Tubiluchus

Class Halicryptomorpha

Order Halicryptomorphida

Family Halicryptidae

Genus Halicryptus

Class Seticoronaria

Order Seticoronarida

Family Maccabeidae

Genus Maccabeus

External links

• Paul Marks (2006-08-09). "X-rays break the silence of ancient worms". New Scientist. http://www.newscientisttech.com/article.ns?id=mg19125645.600&feedId=online-news_rss20. 
• "Evolution of a penis worm". University of Bristol. 2006-08-09. http://www.bris.ac.uk/news/2006/1052.html. 
• Play It Again, Life: Natural History, February 1986

References

• This article incorporates text from the Encyclopædia Britannica, Eleventh Edition, a publication now in the public domain.

v • d • e Opisthokonta: Extant phyla of kingdom Animalia by subkingdom Parazoa Porifera (Calcarea, Demospongiae, Hexactinellida) · Placozoa (Trichoplax) Mesozoa Orthonectida · Rhombozoa Eumetazoa Radiata Ctenophora · Cnidaria (Anthozoa, Hydrozoa, Scyphozoa, Cubozoa, Staurozoa, Myxozoa) Bilateria Protostomia Ecdysozoa Cycloneuralia: Scalidophora (Kinorhyncha, Loricifera, Priapulida) · Nematoida (Nematoda, Nematomorpha) Panarthropoda: Onychophora · Tardigrada · Arthropoda Spiralia Platyzoa Platyhelminthes · Gastrotricha Gnathifera: Rotifera · Acanthocephala · Gnathostomulida · Micrognathozoa · Cycliophora Lophotrochozoa Trochozoa (Sipuncula, Nemertea, Mollusca, Annelida, Echiura) · Brachiozoa Lophophorata (Bryozoa, Entoprocta, Phoronida, Brachiopoda) Deuterostomia Ambulacraria Hemichordata · Echinodermata · Xenoturbellida Chordata Craniata (Vertebrata, Hyperotreti) · Cephalochordata · Tunicata Basal/disputed Acoelomorpha (Acoela, Nemertodermatida) · Chaetognatha

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