[3] In 1814, the French zoologist Constantine Samuel Rafinesque used the word "Sipuncula" to describe the family (now Sipunculidae),[4] and in time, the term came to be used for the whole class.

[7][8] Subsequent analysis of the mitochondrion's DNA has confirmed their close relationship to the Annelida (including echiurans and pogonophorans).

[1] It has also been shown that a rudimentary neural segmentation similar to that of annelids occurs in the early larval stage, even if these traits are absent in the adults.

The sipunculan body is divided into an unsegmented, bulbous trunk and a narrower, anterior section, called the "introvert".

[13] Three genera (Aspidosiphon, Lithacrosiphon and Cloeosiphon) in the Aspidosiphonidae family possess epidermal structures, known as anal and caudal shields.

In Aspidosiphon the shield is a hardened, horny structure; in Lithacrosiphon it is a calcareous cone; in Cloeosiphon it is composed of separate plates.

At the posterior end of the trunk, a hardened caudal shield is sometimes present in Aspidosiphon;[14] this may help with anchoring the animal in its burrow or may be used in the boring process.

A rectal caecum, present in most species, is a blind ending sac at the transition between intestine and rectum with unknown function.

Nitrogenous waste is excreted through a pair of metanephridia opening close to the anus, except in Phascolion and Onchnesoma, which have only a single nephridium.

[11] The tentacular coelom connects the tentacles at the tip of the introvert to a ring canal at their base, from which a contractile vessel runs along beside the esophagus and ends blindly posteriorly.

However, in other species the skin is thin and respiration is mainly through the cuticle of the trunk, where oxygen uptake is assisted by the presence of dermal coelomic canals just beneath the epidermis.

They inhabit a range of habitats including burrowing in sand, mud, clay and gravel, hiding under stones, in rock crevices, in hollow coral heads, in wood, in empty seashells and inside the bones of dead whales.

[20] One species of sipunculan, Themiste lageniformis, has been recorded as reproducing parthenogenetically; eggs produced in the absence of sperm developed through the normal stages.

[10] In at least one species, Themiste pyroides, swarming behaviour occurs with adults creating compact masses among rocks immediately before spawning.

In a few species, the trochophore does not develop directly into the adult, but into an intermediate pelagosphaera stage, that possesses a greatly enlarged metatroch (ciliated band).

One species, Thysanocardia procera is thought to be carnivorous, gaining entrance in some way to the interior of the sea mouse Aphrodita aculeata and sucking out its liquefied contents.

These fossils appear to belong to the crown group,[24][25] and demonstrate that sipunculans have changed little (morphologically) since the early Cambrian, about 520 million years ago.

[24] Some scientists once hypothesized a close relationship between sipunculans and the extinct hyoliths, operculate shells from the Palaeozoic with which they share a helical gut; but this hypothesis has since been discounted.

[28] Sipunculid worm jelly (土笋凍) is a delicacy in southeast China, originally from Anhai, near Quanzhou.

[30] The worms, especially in dried form, are considered a delicacy in Vietnam as well, where they are caught on the coasts of Minh Chao island, in Van Don District.

[30] The relatively high market price of the worms have made them a significant source of income for the local population of fishermen families.