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Penis worm

Priapulida 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.

They were likely major predators of the Cambrian period.

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.

Note:   The above text is excerpted from the Wikipedia article "Penis worm", which has been released under the GNU Free Documentation License.
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