Priapulids Phylum Priapulida
Priapulids are small, yellow or brown, carnivorous worms. They are free-living and partly bury themselves in marine sediments of polar, temperate, and tropical seas worldwide, at depths down to several kilometres! There are only 16 known priapulid species, but at least three of these occur in the Arctic: Priapulus caudatus, Priapulopsis bicaudatus, and Halicryptus spinulusos.
The cylindrical priapulid body, ranging in length from 0.5 mm to 30 cm, is covered by a thin cuticle, which is divided into three regions: a tube-like proboscis, a trunk, and a tail-like caudal appendage. Priapulids move through the sediment by pushing and pulling their body along with the help of layered muscles. Small priapulids are thought to be deposit feeders, consuming bacteria, but large species feed on soft-bodied invertebrates, such as polychaete worms and other priapulids. Their mouth is extended during feeding with their oral teeth being used to seize prey.
Priapulids have separate sexes and juvenile priapulids periodically moult their outer cuticle before becoming an adult. Two closely allied priapulid species, Priapulus caudatus from the Arctic and P. tuberculatospinosus from the Antarctic, are morphologically similar as both species have a thick proboscis and a caudal appendage that resembles a mass of tentacles - but these two species are genetically divergent. Evolution has occurred molecularly, but not morphologically, resulting in two species that look the same, but are very different genetically!