Temporal range: Cambrian–Recent
|Classes, orders and families|
The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a group containing 144–320 species (estimates vary) of bilaterally symmetrical, unsegmented marine worms. Traditionally considered a phylum, they might be a subgroup of phylum Annelida based on recent molecular work.
The first species of this phylum was described in 1827 by the French zoologist Henri Marie Ducrotay de Blainville who named it Sipunculus vulgaris. A related species was later described as Golfingia macintoshii by E. Ray Lankester. The specimen was provided by a friend of his, Professor Mackintosh. The specimen was dissected by Lankester between rounds of golf at Saint Andrews golf club in Scotland from which the species derives its name. Golfingia is now the genus name and Sipuncula the name of the phylum to which these worms belong.
Sipunculids are all marine and are relatively common, and live in shallow waters, either in burrows or in discarded shells like hermit crabs do. Some bore into solid rocks to make a shelter for themselves. Although typically less than 10 cm long, some sipunculans may reach several times that length.
Sipunculans are worm-like animals ranging from 2 to 720 millimetres (0.079 to 28.346 in) in length, with most species being under 10 centimetres (3.9 in). The sipunculan body is divided into an unsegmented trunk and a narrower, retractable anterior section, called the "introvert". Sipunculans have a body wall somewhat similar to that of annelids (though unsegmented) in that it consists of a non-ciliated epidermis overlain by a cuticle, an outer layer of circular and an inner layer of longitudinal musculature. The body wall surrounds the coelom that is filled with fluid on which the body wall musculature acts as a hydrostatic skeleton to extend or contract the animal. When threatened, Sipunculids can retract their body into a shape resembling a peanut kernel—a practice that has given rise to the name "peanut worm". The introvert is retractable into the trunk via two pairs of retractor muscles that extend as narrow ribbons from the trunk wall to attachment points in the introvert. The introvert can be protruded from the trunk by contracting the muscles of the trunk wall, thus forcing the fluid in the body cavity forwards.
The sipunculan mouth is located at the anterior end of the introvert, which is surrounded by a mass of 18–24 ciliated tentacles in the Sipunculidea. In the Phascolosomatidea, the tentacles are arranged in an arc around the nuchal organ, also located at the tip of the introvert. The tentacles are used to gather organic detritus from the water or substrate, and probably also function as gills. The tentacles at the tip of the introvert are hollow and are extended via hydrostatic pressure in a similar manner as the introvert, but have a separate system from that of the rest of the introvert; they are connected, via a system of ducts, to one or two contractile sacs next to the oesophagus. Hooks are often present near the mouth on the introvert. These are proteinaceous, non-chitinous specializations of the epidermis, either arranged in rings or scattered.
Three genera (Aspidosiphon, Lithacrosiphon and Cloeosiphon) possess epidermal modifications, called the anal shield near the anteriorly located anus on the trunk just below the introvert of the animal.' In Aspidosiphon and Lithacrosiphon the anal shield is restricted to the dorsal side, causing the introvert to emerge at an angle, whereas it surrounds the anterior trunk in Cloeosiphon with the introvert emerging from its center. 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, a hardened caudal shield is sometimes present in Aspidosiphon.
The digestive tract of Sipunculans starts with the esophagus, located between the introvert retractor muscles. In the trunk the intestine runs posteriorly, forms a loop and turns anteriorly again. The downward and upward sections of the gut are coiled around each other, forming a double helix. At the anterior end of the gut coil the rectum emerges and ends in the anus. A rectal caecum, present in most species, is a blind ending sac at the transition between intestine and rectum with unknown function. The anus is often not visible when the introvert is retracted into the trunk.
Sipunclans do not have a vascular blood system. Fluid transport and gas exchange are instead accomplished by the coelom, which contains the respiratory pigment haemerythrin, and the tentacular system. The coelomic interstitial fluid contains five types of coelomic cells: haemocytes, granulocytes, large multinuclear cells, ciliated urns and immature cells. The ciliated urn cells may also be attached to the peritoneum and assist in waste filtering from the interstitial fluid. 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. A ciliated funnel, or nephrostome, opens into the coelomic cavity at the anterior end, close to the nephridiopore.
The tentacular system connects the tentacles at the tip of the introvert to a ring canal at their base, from which a contractile vessel that runs along the esophagus and ends blindly posteriorly. Some evidence points towards their involvement in ultrafiltration.
The nervous system consists of a nerve ring (the cerebral ganglion) around the oesophagus, which functions as a brain, and a single ventral nerve cord that runs the length of the body.
In some species, there are simple light-sensitive ocelli associated with the brain. Two organs, likely functioning as a unit for chemoreception are located near the anterior margin of the cerebral ganglion: the non-ciliated cerebral organ, which possesses bipolar sensory cells, and the nuchal organ, located posterior to the cerebral organ. In addition, all sipunculans have numerous sensory nerve endings on the body, especially at the forward end of the introvert.
Both asexual and sexual reproduction can be found in Sipunculans, although asexual reproduction is uncommon. Sipunculans reproduce asexually via transverse fission followed by regeneration of vital body components.
Most sipunculan species are dioecious. Their gametes are produced in the coelomic lining, where they are released into the coelom to mature. These gametes are then picked up by the metanephridia system and released into the aquatic environment, where fertilisation takes place.
Although some species hatch directly into the adult form, many have a trochophore larva, which metamorphoses into the adult after anything from a day to a month, depending on species. 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).
The phylogenetic placement of this phylum in the past has proved troublesome. Originally classified as annelids, despite the complete lack of segmentation, bristles and other annelid characters, the phylum Sipuncula was later allied with the Mollusca, mostly on the basis of developmental and larval characters. Currently these two phyla have been included in a larger group, the Lophotrochozoa, that also includes the annelids, the ribbon worms and several other phyla. Phylogenetic analyses based on 79 ribosomal proteins indicated a position of Sipuncula within Annelida. Subsequent analysis of the mitochondrion's DNA has confirmed their close relationship to the Myzostomida and Annelida (including echiurans and pogonophorans). 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.
Fossils of sipunculans are extremely rare, and are only known from a few genera:
- Archaeogolfingia and Cambrosipunculus) from the Cambrian Chengjiang biota in China. These fossils appear to belong to the crown group, and illustrate that sipunculans have changed little (morphologically) since the early Cambrian, about 520 million years ago.
- An unnamed worm from the Burgess Shale (Stanley Glacier). 
- The Silurian/Carboniferous Lecthaylus.
Some scientists have hypothesized a close relationship between sipunculans and the extinct hyoliths, operculate shells from the Palaeozoic with which they share a helical gut. If this hypothesis is true, the anal plate of some sipunculans might be a remnant of a shell similar to that of hyoliths.
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