Temporal range: Upper Carboniferous–Recent
|Bonellia viridis, female|
The Echiura, or spoon worms, are a small group of marine animals. Once treated as a separate phylum, they are now universally considered to represent derived annelid worms. The Echiura fossilise poorly and the earliest known specimen is from the Upper Carboniferous (Pennsylvanian). However, U-shaped fossil burrows that could be Echiuran have been found dating back to the Cambrian.
Echiurans are marine worms similar in size and habit to sipunculans. 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 dollars. 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. More than 230 species have been described.
Echiurans have a worm-like body with a large flattened proboscis 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 with the prostomium of other annelids.
Compared with other annelids, echiurans have relatively few setae. 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 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 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.
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 coelomic 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.
Echiurans 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.
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 released into the surrounding water through the metanephridia. Fertilization is external.
The species Bonellia viridis, also remarkable for the possible antibiotic properties of bonellin, the green chemical in its skin, is unusual for its extreme sexual dimorphism. Females are typically 8 centimetres (3.1 in) in body length, excluding the proboscis, but the males are only 1 to 3 millimetres (0.039 to 0.118 in) long, and spend their lives within the uterus of the female.
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.
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