Sipuncula

The Sipuncula, also known as the Sipunculida, or peanut worms, are a small phylum of marine worms, or they may be a class in the Annelida.Anderson D.T. 1998. The Annelida and their close relatives. In Anderson D.T. Invertebrate Zoology. Oxford University Press, pp. 183–196.  The phylum has 144–320 species (estimates vary) of these bilaterally symmetrical, unsegmented worms.

Anatomy
Sipunculans are worm-like animals ranging from  in length, with most species being under . The body consists of a narrow front section, called the introvert and a larger trunk section. The introvert is retractable into the trunk.

Sipunculans have a coelom (body cavity), and a body wall somewhat similar to that of annelids. The body wall is strong and muscular. When threatened, Sipunculids can retract their body into a shape resembling a peanut kernel – a practice that has given rise to the name "peanut worm".

Introvert
The most recognizable part of Sipunculan worms is their mouth, which is surrounded by a mass of 18–24 ciliated tentacles, all of which may be inverted into the body. The tentacles are used to gather organic detritus from the water or substrate, and probably also function as gills. The mouth and tentacles lie at the tip of the introvert, which is otherwise decorated with numerous spines or nodules. In contrast, the trunk tends to be relatively smooth.

 References 

Category:Minor phyla
Category:Annelids