Priapulida

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Priapulida
Scientific classification
Kingdom: Animalia
Phylum: Priapulida
Théel, 1906

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).

[edit] Classification

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

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