Phylum Echiura

Phylum Echiura (ek-ee-yur a) (Gr. echis, viper, serpent, oura tail, ida, pl. suffix) consists of about 140 species of marine worms that burrow into mud or sand, live in empty snail shells or sand-dollar tests, or rocky crevices. They are found in all oceans—most commonly in littoral zones of warm waters—but some are found in polar waters or dredged from depths of up to 10,000 m. They vary in length from a few millimeters to 40 or 50 cm.

Echiurans are cylindrical and somewhat sausage shaped. Anterior to the mouth is a flattened, extensible proboscis, which cannot be retracted into the trunk. Echiurans are often called “spoon worms” because of the shape of the contracted proboscis in some species. The nervous system of echiurans is fairly simple with a ventral nerve cord that runs the length of the trunk and continues dorsally into the proboscis.

The proboscis has a ciliated groove leading to the mouth. While they lie buried, the proboscis can extend out over the mud for exploration and deposit-feeding. Most species gather very small particles of detritus and move them along the proboscis by cilia; larger particles are moved by a combination of cilia and muscular action or by muscular action alone. Unwanted particles can be rejected along the route to the mouth. The proboscis is short in some forms and long in others. Bonellia, which is only 8 cm long, can extend its proboscis up to 2 m.

One common form, Urechis (Gr. oura, tail, + echis, viper, serpent), has a very short proboscis and lives in a U-shaped burrow in which it secretes a funnel-shaped mucous net. It pumps water through the net, capturing bacteria and fine particulate material in it. Urechis periodically swallows the food-laden net. Lissomyema (Gr. lissos, smooth,  mys, muscle) lives in empty gastropod shells in which it constructs galleries irrigated by rhythmical pumping of water and feeds on detritus and the organic coating of sand and mud gathered by this process. Cuticle and epithelium, which may be smooth or orna mented with papillae, cover the muscular body wall.

There may be a pair of anterior setae or a row of bristles around the posterior end. The coelom is large. The digestive tract is long and coiled and terminates at the posterior end. A pair of anal sacs may have an excretory and osmoregulatory function. Most echiurans have a closed circulatory system with colorless blood but contain hemoglobin in coelomic corpuscles and certain body cells. There are one to many pairs of nephridia, which serve mainly as gonoducts in some species. Gas exchange probably occurs primarily in the hindgut, which is continually filled and emptied by cloacal irrigation.

In some species sexual dimorphism is pronounced, with the female being much the larger of the two. Bonellia has an extreme sexual dimorphism, and tiny males live on the body of the female or in her nephridia. Determination of sex in Bonellia is very interesting. Free-swimming larvae are sexually undifferentiated. Those that settle on the proboscis of a female become males (1 to 3 mm long). About 20 males are usually found in a single female. Larvae that do not contact a female proboscis metamorphose into females. The stimulus for development into males is apparently a hormone produced by a female’s proboscis.

Sexes are separate, with gonads being produced by special regions of the peritoneum in each sex. Mature sex cells break loose from these gonadal regions and leave the body cavity by way of the nephridia. Fertilization is usually external. Early cleavage and trochophore stages are very similar to those of annelids and sipunculans. The trochophore stage, which may last from a few days to 3 months, according to species, is followed by gradual metamorphosis to a wormlike adult.

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Useful External Links

• Spoon Worms: The Wonders Of Subclass Echiuroidea (Phylum Echiura) by Gordon Ramel
• Phylogeny of Echiura updated, with a revised taxonomy to reflect their placement in Annelida as sister group to Capitellidae by Ryutaro Goto, James Monnington, Marija Sciberras, Isao Hirabayashi, Greg W. Rouse
• The Echiura of Japan: Diversity, Classification, Phylogeny, and Their Associated Fauna by Ryutaro Goto