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.

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.