Peanut worms are a group of over 300 species of worms, classified as a separate Phylum Sipunculida (Phylum Sipuncula, by some authors), called Sipunculids or Sipunculans in English. They have a simple tubular shape, with "a tube within a tube" body plan and the internal organs inside a body cavity (coelom), that is lined by a fine epithelium called peritoneum. They are non-segmented and bilaterally symmetrical; some specialists include the Sipunculids among the Protostomes, with most worms, arthropods, and molluscs. Their earliest appearance on Earth dates back to the Paleozoic Middle Devonian deposits. The Sipunculids are exclusively marine and usually lead a sedentary life on the bottom of the sea. Some, like Sipunculus, bury in sand and mud. Others are rock-boring like Phascolosoma and Parasipidosiphon in the Antilles, and Cloeosiphon in the Indian Ocean near the Maldive Islands. They are common in tropical reef limestone; in Hawaii up to 700 Sipunculids were counted on one square meter of coral rock. The mechanism of rock boring is not known. Although mucus is found in burrows, Sipunculids do not build true tubes like other marine worms.
Sipunculids have no known economic value, but their importance for the interpretation of the history of life on Earth and of the relationships of different invertebrate groups should not be underestimated. They also contribute to the ecological balance in their respective habitats.
The body of a peanut worm is relatively simple, divided into an anterior narrow section called the introvert, and a larger posterior trunk. The introvert is not a proboscis: it can be retracted into the anterior end of the trunk; it represents the head and the anterior part of the worms's body. The mouth, surrounded by an oral disc, is found at the outer tip of the introvert and it is covered by a scalloped fringe, lobes, tentacles, or tentacular lobes, some with grooves lined with cilia. On the outside the introvert may be covered with spines, tubercles, or other small projections. The eversion (extrusion) of this structure is due to the muscle contraction of the body wall and the increased pressure of the fluid in the body cavity (coelom) occurring when feeding. The exact mode of feeding varies with species: some depend on ciliary movement to create currents which bring small particles into the mouth. Dead organic matter (detritus) may be trapped in mucus when the tentacles are placed upon it. Some Sipunculids are carnivores and ingest small animals and microorganisms from the substrate. A Sipunculid found in the North Sea, Golfingia procera, is a predator on an annelid worm of the genus Aphrodite (known as a seamouse): it penetrates its body and sucks out the contents. The introvert invaginates (folds inward so that an outer wall becomes an inner wall), and the ingested food passes into the esophagus, the anterior part of the digestive system which in Sipunculids is U-shaped. The long intestine descends to the posterior end of the trunk and then ascends anteriorly in a twisting spiral. The rectum is short and opens through the anus, usually located at the anterior part of the trunk. There is no blood-vascular system and no gas-exchange organs, but the coelomic fluid contains corpuscles with hemerythrin, a respiratory pigment, like human hemoglobin, but containing copper. It serves to carry oxygen to various parts of the body as the corpuscles move around. Excretion of waste products of metabolism is accomplished by a pair of large sac-like metanephridia, which may be compared in function to our kidneys. These open at the level of the anal opening anteriorly and ventrally. Particulate waste products are picked up by clusters of cells located on the peritoneum and capped by a ciliated cell, called "fixed urns." Some of these "urns" become detached and float in the body fluid in the coelom and help pick up waste products that are to be removed by the metanephridia.
The nervous system resembles that of the annelid worms, but it is not segmented. The "brain" is the dorsal ganglion, an accumulation of nerve cells located over the esophagus, which extend into a single ventral nerve cord.
Sensory cells are abundant especially at the end of the introvert. In some species these may be specialized as chemoreceptors that respond to chemical stimuli, or as a pair of pigmented ocelli, primitive light receptors, located in the brain, or dorsal ganglion.
In peanut worms the sexes are separate, male or female. Respective sex cells form from certain parts of the lining of the body cavity, where retractor muscles of the body wall arise. They are shed into the body cavity (coelom) where they mature into sperms or eggs. These leave the body through the excretory channels of the metanephridia. When males shed sperms into the sea water, the nearby females are stimulated to shed mature eggs, which are fertilized externally. The fertilized eggs begin to divide by spiral cleavage, like in some annelids and molluscs. The development may be direct, into a young adult, or, first, a free swimming larva worm that drops to the bottom. This metamorphosis may take one day as in Golfingia or a month, as in Sipunculus. Asexual reproduction by constriction and separation of the posterior end of the trunk occurs in a few Sipunculids.