Priapulida (priapulid worms, or penis worms) are a phylum of marine worms with an extensible spiny proboscis. Priapulid fossils are known at least as far back as the Middle Cambrian. There nearest relatives are probably Kinorhyncha and Loricifera with which they constitute the taxon Scalidophora. Since 2004 fossil worms have become known that seem to be stem group representatives of that taxon. They are placed in two species in the genus Markuelia.

(from 1911 Encyclopedia Britannica)

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 confinued 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. 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 haemocoel.

The Priapuloidea are dioecious, 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 Platyhelminthes, &c., 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.

Apel, Zeitschr. wiss. Zool. (1885), vol xlii.; Scharif, Quart. foam. Mic. Sd. (i885), vol. xxv.; Ehlers, Zeitschr. wiss. Zool. (i86i), vol. xi.; Schauinsland, Zool. Anz. (i886), vol. ix.; Dc Guerr~e, Mission scientifique du Cap Horn (1891), vol vi.; Michaelsen, Jahrb. Hamburg-A fist. (i888), vol. vi.