The trunk is muscular and the
introvert is extended by the hydrostatic pressure exerted by the contraction of
the body wall muscles against the coelom (see Sipunculus nudus right).
The digestive tract is coiled and U-shaped, and the intestine is long. The nephridia are located at nearly the same level as the anus.
Peanut worm reproduction
The sexes are separate, but reproduction can occur by transverse fission with the hindmost fifth of the parent constricting, then detaching to become a new individual.
It is thought that Sipunculans may
share a common ancestor in the Annelida (though it is important to note that Sipunculids are not segmented worms) or Mollusca or Echiura.
Golfingia elongata, above left, inhabits the lower shores of sandy beaches. It was discovered on the golf course in St. Andrews (Scotland) when a golf ball landed in a small depression on the sand near to one - hence its name.
On the left is Lecathylus gregarius, a fossil peanut worm from the early Silurian, 428 - 423 million years ago, from what is now Blue Island, Illinois in the USA.