Digital Zoology: Priapula

Phylum Priapulida

There are only a dozen or so species in the phylum Priapulida and all are marine animals with most living in the bottom sediments of shallow coastal waters to the abyssal depths. The bilaterally symmetric body is covered with a thin chitinous cuticle that is molted as the animal grows and is divided into two parts--an anterior introvert covered with scalids, posteriorly directed curved spines, and a trunk. In some species the posterior end of the trunk has a retractable caudal appendage that may be important in gas exchange or to assist with burrowing. The rings of the annulated trunk correspond to underlying bands of circular muscle that combine with longitudinal muscles and the fluid-filled body cavity to form the hydrostatic skeleton. When they burrow, the introvert and caudal appendages help to anchor the animal as it digs in the soft sediments. In addition to being a hydraulic medium, the fluid of the body cavity is also the main circulatory fluid. It contains the unusual respiratory pigment hemerythrin and is filtered by protonephridia. Gas exchange occurs across the body wall. Biologists don't agree on whether the body cavity is a true coelom or a pseudocoelom.

Priapulids come in two sizes: large macrobenthic species, with the largest being 32 centimeters long, and small meiobenthic animals less than 0.5 millimeters in length. Macrobenthic species are buried in the sediments with their mouth directed up ready to capture prey with the introvert. They feed on small invertebrates, and when the introvert is retracted, captured prey passes into the straight-tube gut with a terminal anus. Smaller species are interstitial organisms feeding on bacteria and small organisms found in the sediments. In both, a nerve ring surrounds the pharynx and is connected to a single ventral nerve cord that is in turn connected to tactile papillae on the surface of the animal and the caudal appendage that may be important in chemoreception.

The sexes are separate, and paired gonads are connected to the protonephric duct forming a urogenital duct that opens to the outside. Fertilization is external; the first cell divisions involve radial cleavage; and a small larval stage, called a loricate larva because of its resemblance to loriciferans, emerges and undergoes a gradual metamorphosis into a small priapulid worm. Males in one species of priapulids have never been identified, and it's assumed that they reproduce by parthenogenesis.

The Burgess Shale from the Cambrian contains priapulid fossils, and their abundance in the fossil record testifies to the one-time success of the predatory priapulid body plan. Their decline is probably related to the appearance of the jawed polychaete worms that appeared about 50 million years later. Depending on whether their body is a pseudocoelom, the priapulids have been bounced around the taxonomic schemes used for animal phylogeny. A true coelom placed them near the other wormlike animals including echiurans and sipunculids, even though they have radial cleavage during their development. A pseudocoelom would put them with the other vermiform pseudocoelomates. Increasing evidence suggests that priapulids, kinorhynchs, and loricifera are closely related, sharing the characteristic of the scalid-covered introvert, and are part of a larger group of animals that also have an introvert and a brain, or nerve ring, surrounding the pharynx--the nematodes and nematomorphs. In both morphological and molecular interpretations the integrity of the group holds, although where it is placed differs. Because of their molted cuticle and the absence of epidermal cilia, the molecular scheme includes priapulids as part of the larger Ecdysozoa. In the morphological interpretation, the introvert with scalids and a nerve ring may be evidence enough for a single phylum, the Cephaloryncha. There is no common name for the priapulids and that may be related to Priapulida being derived from Priapos, the Greek god of reproduction symbolized by a rather large penis--it may explain why the usual literal translation isn't used as a common name for these worms!