thumb|right|120px|Echiurus
The Echiura, or spoon worms, are a small group of marine animals. Once treated as a separate phylum, they are now considered to belong to Annelida. Annelids typically have their bodies divided into segments, but echiurans have secondarily lost their segmentation. The majority of echiurans live in burrows in soft sediment in shallow water, but some live in rock crevices or under boulders, and there are also deep sea forms. More than 230 species have been described.
Spoon worms are cylindrical, soft-bodied animals usually possessing a non-retractable proboscis which can be rolled into a scoop-shape to feed. In some species the proboscis is ribbon-like, longer than the trunk and may have a forked tip. Spoon worms vary in size from less than a centimetre in length to more than a metre.
Most are deposit feeders, collecting detritus from the sea floor. Fossils of these worms are seldom found and the earliest known fossil specimen is from the Middle Ordovician.
Taxonomy and evolution
The spoonworm Echiurus echiurus was first described by the Prussian naturalist Peter Simon Pallas in 1766; he placed it in the earth worm genus Lumbricus. In the mid-nineteenth century Echiura was placed, alongside Sipuncula and Priapulida, in the now defunct class Gephyrea (meaning a "bridge") in Annelida, because it was believed that they provided a link between annelids and holothurians. In 1898, Sedgwick raised the sipunculids and priapulids to phylum status but considered Echiuroids to be a class of the Annelida. During the early 1900s, a biologist named Jon Stanton Whited devoted his working life to study the echiurans and classify many of its different species. In 1940, after the American marine biologist W. W. Newby had studied the embryology and development of Urechis caupo, he raised the group to phylum status.
They are now universally considered to represent derived annelid worms; as such, their ancestors were segmented worms but echiurans have secondarily lost their segmentation. Their presumed sister group is the Capitellidae.
Having no hard parts, these worms are seldom found as fossils. One of the oldest known unambiguous examples is Coprinoscolex ellogimus from the Mazon Creek fossil beds in Illinois, dating back to the Middle Pennsylvanian period. This exhibits a proboscis, cigar‐shaped body and convoluted gut, and shows that already at that time, echiurans were unsegmented and were essentially similar to modern forms. However, U-shaped burrow fossils that could be Echiuran have been found dating back to the Cambrian, and an Ordovician species of thalassematid named Llwygarua suzannae was found in the Castle Bank lagerstätte.
Anatomy
Spoon worms vary in size from the giant Ikeda taenioides, nearly long with its proboscis extended, to the minute Lissomyema, measuring just . The proboscis has rolled-in margins and a groove on the ventral surface. The distal end is sometimes forked. The proboscis can be very long; in the case of the Japanese species Ikeda taenioides, the proboscis can be long while the body is only . Even smaller species like Bonellia can have a proboscis a metre (yard) long. The proboscis is used primarily for feeding. Respiration takes place through the proboscis and the body wall, with some larger species also using cloacal irrigation. In this process, water is pumped into and out of the rear end of the gut through the anus.
Compared with other annelids, echiurans have relatively few setae (bristles). In most species, there are just two, located on the underside of the body just behind the proboscis, and often hooked. In others, such as Echiurus, there are also further setae near the posterior end of the animal. Unlike most annelids, adult echiurans have no trace of segmentation.
The body wall is muscular. It surrounds a large coelom which leads to a long looped intestine with an anus at the rear tip of the body. The intestine is highly coiled, giving it a considerable length in relation to the size of the animal. A pair of simple or branched diverticula are connected to the rectum. These are lined with numerous minute ciliated funnels that open directly into the body cavity, and are presumed to be excretory organs. They often congregate in sediments with high concentrations of organic matter. One species, Lissomyema mellita, which lives off the southeastern coast of the US, inhabits the tests (exoskeleton) of dead sand dollars. When the worm is very small, it enters the test and later becomes too large to leave.
In the 1970s, the spoon worm Listriolobus pelodes was found on the continental shelf off Los Angeles in numbers of up to 1,500 per square metre (11 square feet) near sewage outlets.
Behaviour
A spoon worm can move about on the surface by extending its proboscis and grasping some object before pulling its body forward. Some worms, such as Echiurus, can leave the substrate entirely, swimming by use of the proboscis and contractions of the body wall.
Digging behaviour has been studied in Echiurus echiurus. When burrowing, the proboscis is raised and folded backwards and plays no part in the digging process. The front of the trunk is shaped into a wedge and pushed forward, with the two anterior chaetae (hooked chitinous bristles) being driven into the sediment. Next the rear end of the trunk is drawn forward and the posterior chaetae anchor it in place. These manoeuvres are repeated and the worm slowly digs its way forwards and downwards. It takes about forty minutes for the worm to disappear from view. The burrow descends diagonally and then flattens out, and it may be a metre or so long before ascending vertically to the surface.
thumb|right|Characteristic forked proboscis of an echiurian worm in the Maldives
thumb|[[Bonellia viridis, female]]
Spoon worms are typically detritivores, extending the flexible and mobile proboscis and gathering organic particles that are within reach. Some species can expand the proboscis by ten times its contracted length. The proboscis is moved by the action of cilia on the lower (ventral) surface "creeping" it forward. When food particles are encountered, the sides of the proboscis curl inward to form a ciliated channel. It has a short proboscis and a ring of mucous glands at the front of its body. It expands its muscular body wall to deposit a ring of mucus on the burrow wall then retreats backwards, exuding mucus as it goes and spinning a mucus net. It then draws water through the burrow by peristaltic contractions and food particles stick to the net. When this is sufficiently clogged up, the spoon worm moves forward along its burrow devouring the net and the trapped particles. This process is then repeated and in a nutrient-rich area may take only a few minutes to complete. Large particles are squeezed out of the net and eaten by other invertebrates living commensally in the burrow. These typically include a small crab, a scale worm and often a fish lurking just inside the back entrance. Other spoon worms live concealed in rock crevices, empty gastropod shells, sand dollar tests and similar places, extending their proboscises into the open water to feed.
While the proboscis of a burrowing spoon worm is on the surface it is at risk of predation by bottom-feeding fish. In some species, the proboscis will autotomise (break off) if attacked and the worm will regenerate a proboscis over the course of a few weeks.
Reproduction
Echiurans are dioecious, with separate male and female individuals. The gonads are associated with the peritoneal membrane lining the body cavity, into which they release the gametes. The sperm and eggs complete their maturation in the body cavity, before being stored in genital sacs, which are specialised metanephridia. At spawning time, the genital sacs contract and the gametes are squeezed into the water column through pores on the worm's ventral surface. Fertilization is external.
Fertilized eggs hatch into free-swimming trochophore larvae. In some species, the larva briefly develops a segmented body before transforming into the adult body plan, supporting the theory that echiurans evolved from segmented ancestors resembling more typical annelids. They are also eaten as a fermented product known as gaebul-jeot.
List of families
According to the World Register of Marine Species:
- family Bonelliidae <small>Lacaze-Duthiers, 1858</small>
- family Ikedidae <small>Bock, 1942</small>
- suborder Echiurida
- family Echiuridae <small>Quatrefages, 1847</small>
- family Thalassematidae <small>Forbes & Goodsir, 1841</small>
- family Urechidae <small>Fisher & Macginitie, 1928</small>
<gallery style="text-align:center;" mode="packed">
Image:Un ver de la famille des Bonelliidae.jpg|A worm of the family Bonelliidae
Image:Bonelliidae à identifier.jpg|Ochetostoma erythrogrammon, family Echiuridae
Image:Arhynchite hayaoi - ZooKeys-312-013-g001.jpeg|Arhynchite hayaoi, family Thalassematidae
Image:Urechiscaupo.jpg|Urechis unicinctus, family Urechidae
</gallery>