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The Sipuncula or Sipunculida, sipunculid worms or peanut worms, are a phylum containing 144-320 species
Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring....
 (estimates vary) of bilaterally symmetrical, unsegmented
Segmentation (biology)

Segmentation in biology refers to the division of some metazoan bodies and plant body plans into a series of semi-repetitive segments, and the question of the benefits and costs of doing so....
 marine
SEA

See also: Sea and seasThe three-letter acronym SEA may refer to:People/organizations/businesses*Scientists and Engineers for America, a pro-science political advocacy group....
 worms. Sipunculid worm jelly is a delicacy in the town of Xiamen
Xiamen

Xiamen, also known as Amoy , is a coastal sub-provincial city in southeastern Fujian province of China, People's Republic of China. It looks out to the Taiwan Strait and borders Quanzhou to the north and Zhangzhou to the south....
 in Fujian province of China
China

China is a Culture of China, an ancient civilization, and, depending on perspective, a national or multinational entity extending over a large area in East Asia....
.

nculids are relatively common, and live in shallow waters, either in burrow
Burrow

A burrow is a hole or tunnel dug into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion....
s or in discarded shells like hermit crab
Hermit crab

Hermit crabs are Decapoda crustaceans of the superfamily Paguroidea . They are not closely related to true crabs. Hermit crabs are quite commonly seen in the intertidal zone, for example in tide pools....
s do.






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The Sipuncula or Sipunculida, sipunculid worms or peanut worms, are a phylum containing 144-320 species
Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring....
 (estimates vary) of bilaterally symmetrical, unsegmented
Segmentation (biology)

Segmentation in biology refers to the division of some metazoan bodies and plant body plans into a series of semi-repetitive segments, and the question of the benefits and costs of doing so....
 marine
SEA

See also: Sea and seasThe three-letter acronym SEA may refer to:People/organizations/businesses*Scientists and Engineers for America, a pro-science political advocacy group....
 worms. Sipunculid worm jelly is a delicacy in the town of Xiamen
Xiamen

Xiamen, also known as Amoy , is a coastal sub-provincial city in southeastern Fujian province of China, People's Republic of China. It looks out to the Taiwan Strait and borders Quanzhou to the north and Zhangzhou to the south....
 in Fujian province of China
China

China is a Culture of China, an ancient civilization, and, depending on perspective, a national or multinational entity extending over a large area in East Asia....
.

Habitat

Sipunculids are relatively common, and live in shallow waters, either in burrow
Burrow

A burrow is a hole or tunnel dug into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion....
s or in discarded shells like hermit crab
Hermit crab

Hermit crabs are Decapoda crustaceans of the superfamily Paguroidea . They are not closely related to true crabs. Hermit crabs are quite commonly seen in the intertidal zone, for example in tide pools....
s do. Some bore into solid rocks to make a shelter for themselves. Although typically less than 10 cm
Centimetre

A centimetre is a Units of measurement of length in the metric system, equal to one hundredth of a metre, which is the current International System of Units SI base unit of length....
 long, some sipunculans may reach several times that length.

Anatomy

The most recognizable part of Sipunculan worms is their mouth, which is surrounded by a mass of 18 - 24 tentacle
Tentacle

Tentacles can refer to the elongated flexible organs that are present in some animals, especially invertebrates, and sometimes to the hairs of the leaves of some carnivorous plant....
s, all of which may be inverted into the body. There is no segmentation or septa present in Sipunculans. Their body consists of an introvert and a trunk, the introvert being retractable into the trunk. The digestive tract of Sipunculans passes from the mouth to the posterior end of the body, before twisting back around itself and ending at the anus
Anus

The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest, such as coprolite ; food material after all the nutrients have b...
, on the dorsal side of the body. The anus is often not visible when the introvert is retracted into the trunk. A few taxa possess a calcified plate called the anal shield. Sipunculans have a coelom
Body cavity

By the broadest definition, a body cavity is any fluid filled space in a multicellular organism. However, the term usually refers to the space, located between an animal?s outer covering and the outer lining of the gut cavity, where internal organs develop....
. However, they do not have a vascular blood system. Instead, interstitial fluid
Interstitial fluid

Interstitial fluid is a solution which bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes Blood plasma and transcellular fluid....
 transports oxygen
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
 and nutrient
Nutrient

A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment....
s around the body. A separate cavity fills the hollow tentacles; it passes oxygen from the tentacles to the coelom. The body wall is strong and muscular
MUSCLE

MUSCLE is public domain, multiple sequence alignment software for protein and nucleotide sequences.MUSCLE is integrated into UGENE bioinformatics tool as a plugin....
; when threatened, Sipunculids can retract their body into a shape resembling a peanut
Peanut

The peanut, or groundnut , is a species in the legume Fabaceae native to South America, Mexico and Central America. It is an annual plant herbaceous plant growing to 30 to 50 cm tall....
 kernel
Seed

A seed is a small Plant embryogenesis plant enclosed in a covering called the seed coat, usually with some Food storage. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant....
. This is where the name "Australian peanut worm" comes from.

Reproduction

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. As for sexual reproduction, Sipunculans 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. Fertilization in Sipunculans is external. Once male and female matured gametes meet, a Trochopore larva develops followed by a Pelagosphera larva to a juvenile and finally into an adult.

Relationships

The phylogenetic
Phylogenetics

In biology, phylogenetics is the study of evolutionary relatedness among various groups of organisms , which is discovered through molecular sequencing data and morphological data matrices....
 placement of this phylum has proved troublesome. Originally classified as annelid
Annelid

The annelids, collectively called Annelida , are a large Scientific classification of animals comprising the segmented worms, with about 15,000 modern species including the well-known earthworms and leeches....
s, despite the complete lack of segmentation, bristle
Bristle

A bristle is a stiff hair or feather. Also used are synthetic materials such as nylon in items such as brooms and sweepers. Bristles are often used to make brushes for cleaning uses, as they are strongly abrasive; common examples include the toothbrush and toilet brush....
s and other annelid characters, the phylum Sipuncula was later allied with the Mollusca
Mollusca

MolluscsSpelled mollusk in the USA; the spelling "mollusc" is preferred by some authors, see the reasons given by . are animals belonging to the Phylum Mollusca....
, mostly from development
Morphogenesis

Morphogenesis , is the physical process that gives rise to the shape of an organism. It is one of three fundamental aspects of developmental biology along with the control of cell growth and cellular differentiation....
al and larva
Larva

A larva is a young form of animal with indirect developmental biology, going through or undergoing metamorphosis .The larva can look completely different from the adult form, for example, a caterpillar differs from a butterfly....
l characters. Nowadays, these two phyla are generally included in a larger group, the Lophotrochozoa
Lophotrochozoa

The Lophotrochozoa are one of three major groupings of protostome animals. The taxon was introduced in 1995 in a paper by Kenneth M Halanych et al based on molecular data....
, that also includes annelid
Annelid

The annelids, collectively called Annelida , are a large Scientific classification of animals comprising the segmented worms, with about 15,000 modern species including the well-known earthworms and leeches....
s, ribbon worms
Nemertea

Nemertea is a phylum of invertebrate animals also known as ribbon worms or proboscis worms. Most of the 1,400 or so species are marine, with a few living in fresh water and a small number of terrestrial animal; they are found in all marine habits, and throughout the world's oceans....
, and 4 other phyla.

Fossil record

The fossil record of the Sipuncula is, not surprisingly for soft-bodied animals, sparse. Some scientists, however, consider that hyoliths, operculate
Operculum (gastropod)

The operculum, meaning little lid, is a corneous or calcareous structure which exists in some groups of marine, freshwater, but not in terrestrial snails or gastropods....
 shells from the Palaeozoic may be related to the sipunculids, with the only remnant of the shell in extant forms being the anal plate.

Even excluding hyoliths, fossils of sipunculans are known extending back to the Cambrian
Cambrian

The Cambrian is a geologic period that began about Mya at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period ....
 Period. Fossils of the genera Archaeogolfingia and Cambrosipunculus from China are not drastically different from members of the Sipunculidea living today.

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