Priapulida (priapulid worms or penis worms, from Gr. πριάπος, priāpos 'Priapus' + Lat. -ul-, diminutive) is a phylum of marine worms. They are named for their extensible spiny proboscis, which, in some species, may have a shape similar to that of a human penis. They live in the mud, which they eat, in comparatively shallow waters up to 90 metres (300 ft).
Their nearest relatives are probably Kinorhyncha and Loricifera with which they constitute the taxon Scalidophora. Besides arthropods and velvet worms, the priapulids are the only members of the Ecdysozoa which are relatively large in size. They feed on slow-moving invertebrates, such as polychaete worms.
Priapulid-like fossils are known at least as far back as the Middle Cambrian. They were likely major predators of the Cambrian period. However crown-group priapulids cannot be recognized until the Carboniferous. There are only sixteen known extant species of priapulid worms.
Priapulds are cylindrical worm-like animals, ranging from 0.5 to 20 centimetres (0.20 to 7.9 in) in length, with a median anterior mouth quite devoid of any armature or tentacles. The body is divided into a main trunk or abdomen, and a somewhat swollen proboscis region ornamented with longitudinal ridges. The body is ringed, and often has circles of spines, which are continued into the slightly protrusible pharynx. Some species may also have a tail or a pair of caudal appendages. The body wall is lined by a chitinous cuticle that is moulted as the animal grows.
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. Although there are no vascular or respiratory systems, the body cavity does contain phagocytic amoebocytes and cells containing the respiratory pigment haemerythrin.
The alimentary canal is straight, consisting of an eversible pharynx, an intestine, and a short rectum. The pharynx is muscular and lined by teeth. The anus is terminal, although in Priapulus one or two hollow ventral diverticula of the body-wall stretch out behind it.
The nervous system is composed of a nerve ring around the pharynx and a single ventral cord running the length of the body and including a number of ganglia. It retains its primitive connection with the ectoderm, forming part of the body wall. There are no specialized sense organs, although there are sensory nerve endings in the body, especially on the proboscis.
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