Introduction to Sipuncula
The Peanut Worms
Sipunculids from Ano Nuevo Park, California
Sipuncula -- These animals, which are commonly called "peanut worms" because some have the general shape of shelled peanuts, are not particularly well studied. Only about 320 species have been formally described, all marine and mostly from shallow waters. While some (like the specimen of Sipunculus shown below) burrow into sand and mud, others (like this unidentified species from French Polynesia) live in crevices in rocks, or in empty shells. Still others bore into rock.
General Sipunculan Anatomy -- The body is completely unsegmented, and the intestine forms a twisted loop, with the anus on the side of the body. Typical of sipunculans is a forward (anterior) body section, the introvert, which can be retracted into the body by the retractor muscles. At the tip of the introvert (retracted in the specimen shown at the top of this page) is the mouth, which is surrounded by a ring of tentacles. The body cavity, or coelom, of sipunculans is large and filled with fluid, in which are found free-floating cells known as hemerythrocytes as well as free-floating clusters of cells known as ciliary urns. Sipunculans have no circulatory or respiratory systems; the coelomic fluid transports both nutrients and oxygen to all parts of the body. Nephridia filter the coelomic fluid.
Sipunculus sp., a burrowing sipunculid
Sipunculan Phylogeny and Systematics -- Sipunculans are sometimes thought to be a group of annelids, and have been classified within them. However, this phylogenetic hypothesis is contradicted by comparing the anatomy of annelids, sipunculans, and other animals. For instance, sipunculans have no trace of important annelid characters such as segmentation and chaetae (bristles). The characters they do share with annelids (e.g. worm shape, introvert, trochophore larva) are not restricted to annelids and sipunculans, but are much more general.

An alternative hypothesis places the sipunculans closer to the molluscs -- the snails, clams, squids, octopuses, and so on. This seems unusual at first glance; sipunculans do not look anything like most molluscs, at least outwardly. However, sipunculans and molluscs share several characters in early development; for example, after fertilization and division of eggs, both molluscs and sipunculans have a characteristic arrangement of cells in the embryo, known as the "molluscan cross."

A third hypothesis considers the characters shared by molluscs and sipunculans to be primitive characters for a larger group known as the Trochozoa, which also includes the annelids.

The Fossil Record of Sipuncula -- Sipunculans have an extremely sparse fossil record -- with one possible exception. A group of fossils known as hyoliths is found in rocks through much of the Paleozoic. Hyoliths are conical shells; each conical shell bears a hinged "lid", or operculum, covering the opening. While these shells somewhat resemble molluscan shells, a few fossil hyoliths have been found that show traces of the intestine -- and the hyolith intestine is looped and coiled, much like that of living sipunculans. A few living sipunculans secrete a calcified cuticular plate, the anal shield, so it is not impossible that past relatives of the sipunculans secreted more extensive shells.