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Priapulida (priapulid worms or penis worms, from Gr. 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 the 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 Mouth, surrounded by spines, 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).

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

• Paul Marks. "X-rays break the silence of ancient worms", New Scientist, 2006-08-09. 
• "Evolution of a penis worm", University of Bristol, 2006-08-09. 

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