The genus Priapulida consists of a few species of small worm-like creatures. Their bodies consist of a proboscis and trunk and sometimes one or two caudal appendages. Their body is covered with a chitinous cuticle with tubercles and spines. They live in the sand or mud on the sea floor.
Some scientists, past and present, did not place the priapulids in a separate phylum. At one time, they were united with other more or less similar groups in the taxon Gephyrea. Others have been inclined to place them with the roundworms, annelids, or even echinoderms, according to Invertebrate Zoology.
The name Priapulida is derived from Priapos, the Greek fertility god who supposedly presided over gardens, vineyards, and agriculture in general. The phylum received this name because these creatures bear a superficial resemblance to the male generative organ.
The discovery of priapulid species is apparently an ongoing process. The oldest source that I consulted claims that there are only six species, while the latest sources recognize fifteen species. This does not include fossils.
Most priapulids live in cold seas, though there are exceptions. For example, Tubiluchus arcticus lives in the White Sea and other Arctic waters, but Tubiluchus remanei lives in the Red Sea, according to the Encyclopedia of Life.
Priapulids like to bury themselves in mud or sand with only the mouth protruding into the water. Some occur near shore, while others live in deeper waters. For example, Tubiluchus corallicola is found at depths ranging from 3.6 to 18 meters, while the depth range of Priapulus abyssorum is 4130 to 4823, according to the Encyclopedia of Life.
The bodies of priapulids are cylindrical in shape. Most sources give 15 centimeters as their maximum length, but Biology of the Invertebrates says that some species can reach 20 centimeters in length.
The main parts of the priapulid body are the proboscis (or presoma) and the trunk. Some species also have one or two caudal appendages.
Priapulids are capable of unusual movements. They can retract their proboscis into their trunk. They have a mouth at the front end of their proboscis, which they can turn inside out, revealing spines with which they grab their food. They also can contract their body. This is the way they burrow through the sand or mud.