Sipuncula
Encyclopedia
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, molecular work suggests that they might be a subgroup of phylum Annelida.
at Saint Andrew
s 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.
s or in discarded shells like hermit crab
s 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.
s (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 tentacle
s 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 gill
s. 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
which are 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.
is often not visible when the introvert is retracted into the trunk.
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
.
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.
of vital body components.
Most sipunculan species are dioecious
. Their gamete
s 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).
placement of this phylum in the past has proved troublesome. Originally classified as annelid
s, despite the complete lack of segmentation, bristle
s and other annelid characters, the phylum Sipuncula was later allied with the Mollusca
, mostly on the basis of development
al and larva
l characters. Currently these two phyla have been included in a larger group, the Lophotrochozoa
, that also includes the annelid
s, 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 echiura
ns and pogonophora
ns).
Period in China. These fossils illustrate that sipunculans have changed little since the early Cambrian.
Some scientists consider a close relationship of sipunculans and the extinct hyoliths, operculate
shells from the Palaeozoic, with which they share the helically twisted gut. If true, the anal plate of some sipunculans might be a remnant of a shell similar to that of hyoliths.
in Fujian province of China
.
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. While in many cases this definition is adequate, more precise or differing measures are...
(estimates vary) of bilaterally symmetrical, unsegmented
Segmentation (biology)
Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...
marine
Sea
A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, it means a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean...
worms. Traditionally considered a phylum, molecular work suggests that they might be a subgroup of phylum Annelida.
History
The first species of this phylum was described in 1827 by the French zoologist Henri Marie Ducrotay de Blainville who named it the name 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 golfGolf
Golf is a precision club and ball sport, in which competing players use many types of clubs to hit balls into a series of holes on a golf course using the fewest number of strokes....
at Saint Andrew
Saint Andrew
Saint Andrew , called in the Orthodox tradition Prōtoklētos, or the First-called, is a Christian Apostle and the brother of Saint Peter. The name "Andrew" , like other Greek names, appears to have been common among the Jews from the 3rd or 2nd century BC. No Hebrew or Aramaic name is recorded for him...
s golf club in Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
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.
Habitat
Sipunculids are all marine and are relatively common, and live in shallow waters, either in burrowBurrow
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. Burrows provide a form of shelter against predation and exposure to the elements, so the burrowing way of life is quite popular among the...
s or in discarded shells like hermit crab
Hermit crab
Hermit crabs are decapod crustaceans of the superfamily Paguroidea. Most of the 1100 species possess an asymmetrical abdomen which is concealed in an empty gastropod shell that is carried around by the hermit crab.-Description:...
s do. Some bore into solid rocks to make a shelter for themselves. Although typically less than 10 cm
Centimetre
A centimetre is a unit of length in the metric system, equal to one hundredth of a metre, which is the SI base unit of length. Centi is the SI prefix for a factor of . Hence a centimetre can be written as or — meaning or respectively...
long, some sipunculans may reach several times that length.
Anatomy
Sipunculans are worm-like animals ranging from 2 to 720 mm (0.078740157480315 to 28.3 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 annelidAnnelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s (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
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
. The body wall surrounds the 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...
that is filled with fluid on which the body wall musculature acts as a hydrostatic skeleton
Hydrostatic skeleton
A hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles. The pressure of the fluid and action of the surrounding circular and longitudinal muscles are used to change an...
to extend or contract the animal. When threatened, Sipunculids can retract their body into a shape resembling a peanut
Peanut
The peanut, or groundnut , is a species in the legume or "bean" family , so it is not a nut. The peanut was probably first cultivated in the valleys of Peru. It is an annual herbaceous plant growing tall...
kernel
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
- 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 tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...
s 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 gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s. 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
Epidermis (zoology)
The Epidermis is an epithelium that covers the body of an eumetazoan . Eumetazoa have a cavity lined with a similar epithelium, the gastrodermis, which forms a boundary with the epidermis at the mouth.Sponges have no epithelium, and therefore no epidermis or gastrodermis...
which are 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.
Digestive System
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 anusAnus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of 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,...
is often not visible when the introvert is retracted into the trunk.
Circulation
Sipunclans do not have a vascular bloodBlood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
system. Fluid transport and gas exchange are instead accomplished by the coelom
Coelom
The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
which contains the respiratory pigment haemerythrin, and the tentacular system. The coelomic interstitial fluid
Interstitial fluid
Interstitial fluid is a solution that bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes plasma and transcellular 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
Peritoneum
The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...
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
Ultrafiltration (renal)
In biological terms, ultrafiltration occurs at the barrier between the blood and the filtrate in the renal corpuscle or Bowman's capsule in the kidneys. The Bowman's capsule contains a dense capillary network called the glomerulus. Blood flows into these capillaries through a wide afferent...
.
Nervous system
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.
Reproduction
Both asexual and sexual reproduction can be found in Sipunculans, although asexual reproduction is uncommon. Sipunculans reproduce asexually via transverse fission followed by regenerationRegeneration (biology)
In biology, regeneration is the process of renewal, restoration, and growth that makes genomes, cells, organs, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of regeneration, from bacteria to humans. At its most...
of vital body components.
Most sipunculan species are dioecious
Dioecious
Dioecy is the property of a group of biological organisms that have males and females, but not members that have organs of both sexes at the same time. I.e., those whose individual members can usually produce only one type of gamete; each individual organism is thus distinctly female or male...
. Their gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s 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
Trochophore
A trochophore is a type of free-swimming planktonic marine larva with several bands of cilia.By moving their cilia rapidly, a water eddy is created. In this way they control the direction of their movement...
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).
Relationships
The phylogeneticPhylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
placement of this phylum in the past has proved troublesome. Originally classified as annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, 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
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...
, mostly on the basis of development
Morphogenesis
Morphogenesis , is the biological process that causes an organism to develop its shape...
al and larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
l characters. Currently these two phyla have been included in a larger group, the Lophotrochozoa
Lophotrochozoa
The Lophotrochozoa are a major grouping of protostome animals. The taxon was discovered based on molecular data. Molecular evidence such as a result of studies of the evolution of small-subunit ribosomal RNA supports the monophyly of the phyla listed in the infobox shown at right.-Terminology:The...
, that also includes the annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s, the ribbon worms
Nemertea
Nemertea is a phylum of invertebrate animals also known as "ribbon worms" or "proboscis worms". Alternative names for the phylum have included Nemertini, Nemertinea and Rhynchocoela. Although most are less than long, one specimen has been estimated at , which would make it the longest animal ever...
and several other phyla. Phylogenetic analyses based on 79 ribosomal proteins indicated a position of Sipuncula within Annelida. Subsequent analysis of the mitochondrion
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
's DNA has confirmed their close relationship to the Myzostomida
Myzostomida
Myzostomida are a remarkable taxonomic group of small marine worms, which are parasitic on crinoids or "sea lilies", a kind of echinoderm. These unusual parasitic Lophotrochozoa were first discovered by Friedrich Sigismund Leuckart in 1827....
and Annelida (including echiura
Echiura
The Echiura, or spoon worms, are a small group of marine animals. They are often considered to be a group of annelids, although they lack the segmented structure found in other members of that group, and so may also be treated as a separate phylum...
ns and pogonophora
Pogonophora
There are two taxa with the name Pogonophora:*Pogonophora - an obsolete animal phylum, now treated as part of the family Siboglinidae*Pogonophora - a genus in the Euphorbiaceae...
ns).
Fossil record
Fossils of sipunculans are extremely rare, and are only known from two genera (Archaeogolfingia and Cambrosipunculus) of the CambrianCambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
Period in China. These fossils illustrate that sipunculans have changed little since the early Cambrian.
Some scientists consider a close relationship of sipunculans and the extinct hyoliths, operculate
Operculum (gastropod)
The operculum, meaning little lid, is a corneous or calcareous anatomical structure which exists in many groups of sea snails and freshwater snails, and also in a few groups of land snails...
shells from the Palaeozoic, with which they share the helically twisted gut. If true, the anal plate of some sipunculans might be a remnant of a shell similar to that of hyoliths.
Edibility
Sipunculid worm jelly (土笋冻) is a delicacy in the town of XiamenXiamen
Xiamen , also known as Amoy , is a major city on the southeast coast of the People's Republic of China. It is administered as a sub-provincial city of Fujian province with an area of and population of 3.53 million...
in Fujian province of China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
.