The Echiura, or spoon worms, are a small group of marine
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s. They are often considered to be a group of 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, although they lack the segmented structure found in other members of that group, and so may also be treated as a separate phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

. However, phylogenetic analyses of DNA sequences place echiurans and pogonophorans
Siboglinidae, also known as the beard worms, is a family of polychaete annelid worms whose members made up the former phyla Pogonophora and Vestimentifera. They are composed of about 100 species of vermiform creatures and live in thin tubes buried in sediments at ocean depths from 100 to 10,000 m...

 within the Annelida.
The Echiura fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

ise poorly and the earliest known specimen is from the Upper Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 (called the Pennsylvanian
The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...

 in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

). However, U-shaped fossil burrows that could be Echiuran have been found dating back to the Cambrian
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...


Echiurans are marine worms similar in size and habit to sipuncula
The Sipuncula or Sipunculida is a group containing 144-320 species of bilaterally symmetrical, unsegmented marine worms...

ns. Many genera, such as Echiurus, Urechis, and Ikeda, live in burrows in sand and mud; others live in rock and coral crevices. One species, Thalassema mellita, which lives off the southeastern coast of the US, inhabits the tests (exoskeleton) of dead sand dollar
Sand dollar
The term Sand dollar refers to species of extremely flattened, burrowing echinoids belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are known as sea biscuits...

s. When the worm is very small, it enters the test and later becomes too large to leave.

The majority of echiurans live in shallow water, but there are also deep sea forms. About 140 species have been described.


Echiurans have a worm-like body with a large flattened proboscis
A proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In simpler terms, a proboscis is the straw-like mouth found in several varieties of species.-Etymology:...

 projecting forward from the head. The body is typically drab in colour, but bright red and green species are known. The proboscis is a sheet-like structure, rolled around into a cylindrical tube with an open gutter at the ventral surface. The length of the proboscis varies greatly between species, and in some species is many times longer than the rest of the body. It is probably homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 with the prostomium
Prostomium is the first body segment in annelid worms. It is in front of the mouth, being usually a small shelf- or lip-like extension over the dorsal side of the mouth. It sometimes bears antennae and eyes. It often functions like a kind of overlip when the animal is feeding...

 of other annelids.

Compared with other annelids, echiurans have relatively few setae
Seta is a biological term derived from the Latin word for "bristle". It refers to a number of different bristle- or hair-like structures on living organisms.-Animal setae:In zoology, most "setae" occur in invertebrates....

. In most species, there are just two, located on the underside of the body just behind the proboscis. In others, such as Echiurus, there are also further setae near the posterior end of the animal. Unlike other annelids, adult echiurans have no trace of segmentation.

The digestive system consists of a simple tube running the length of the body, with the anus
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,...

 being at the posterior end. The tube, however, is highly coiled, giving it a considerable length in relation to the size of the animal. A pair of simple or branched diverticula
A diverticulum is medical or biological term for an outpouching of a hollow structure in the body. Depending upon which layers of the structure are involved, they are described as being either true or false....

 are connected to the rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...

. These are lined with numerous minute ciliated funnels that open directly into the body cavity, and are presumed to be excretory organs.

Although some species lack a blood vascular system, where it is present, it resembles that of other annelids. The blood is essentially colourless, although some haemoglobin-containing cells are present in the 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...

ic fluid of the main body cavity. There can be anything from one to over a hundred metanephridia for excreting nitrogenous waste, which typically open near the anterior end of the animal.

Echiruans do not have a distinct respiratory system, absorbing oxygen through the body wall.

The nervous system consists of a brain near the base of the proboscis, and a ventral nerve cord running the length of the body. Aside from the absence of segmentation, this is a similar arrangement to that of other annelids. Echiurans do not have any eyes or other distinct sense organs.


Typical spoon worms, including Bonellia, are suspension feeders, projecting their proboscis out of their burrows, with the gutter projecting upwards. Edible particles will then settle onto the proboscis and a ciliated channel conducts the food to the trunk.

Perhaps the most remarkable feeding adaptations among the spoon worms can be seen in Urechis. U. caupo lives in a large, U-shaped burrow and by pulsating its body it drives water through its lair. To feed, it produces a conical mucus net that lines the burrow as water is sucked in at a rate of about 18L per hour. Edible particles are caught on the net, and after some time the worm slowly eats the net and all the edible matter sticking to it.


Echiruans are 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...

, with separate male and female individuals. The gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

s are associated with the peritoneal
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...

 membrane lining the body cavity, into which they release the gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...

s. The sperm and eggs complete their maturation in the body cavity, before being released into the surrounding water through the metanephridia. Fertilisation is external.

The species Bonellia viridis
Bonellia viridis
The Green Spoonworm is a marine worm noted for displaying exceptional sexual dimorphism and for the biocidal properties of a pigment in its skin....

, also remarkable for the possible antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

 properties of bonellin, the green chemical in its skin, is unusual for its extreme sexual dimorphism
Sexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...

. Females are typically 8 centimetres (3.1 in) in body length, excluding the proboscis, but the males are only 1 to 3 mm (0.0393700787401575 to 0.118110236220472 in) long, and spend their lives within the uterus of the female.

Fertilized eggs hatch into free-swimming 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...

larvae. In some species, the larva briefly develops a segmented body before transforming into the adult body plan, supporting the theory that echiruans evolved from segmented ancestors resembling more typical annelids.
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