Phylum Echiura

Phylum Echiura
Phylum Echiura (ek-ee-yur´a) (Gr. echis, viper, serpent, + oura tail, + ida, pl. suffix) consists 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 2000 m. They vary in length from a few millimeters to 40 or 50 cm.

Echiurans have only about onethird as many species (140) as sipunculans. There are two classes: Echiurida and Sactosomatida. Echiurida is much larger and includes two orders and five families.

The body of echiurans is cylindrical and somewhat sausage shaped (Figure 21-3). Anterior to the mouth is a flattened, extensible proboscis which, unlike that of sipunculids, cannot be retracted into the trunk. Echiurids are often called “spoonworms” because of the shape of the contracted proboscis in some species. The proboscis, which contains the brain, is actually a cephalic lobe, probably homologous to the annelid prostomium. The proboscis has a ciliated groove leading to the mouth. While the animal lies buried, the proboscis can extend out over the mud for exploration and deposit feeding (Figure 21-4). Bonellia viridis picks up very small particles and moves 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), 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 sand and mud drawn in by the irrigation process.



Cuticle and epithelium, which may be smooth or ornamented with papillae (Figure 21-3), 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 (Figure 21-5). 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. Two to many nephridia serve mainly as gonoducts. A nerve ring runs around the pharynx and forward into the proboscis, and there is a ventral nerve cord. There are no specialized sense organs.

Sexes are separate, with a single gonad in each sex. Mature sex cells break loose from the gonads and leave the body cavity by way of the nephridia, and 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 the wormlike adult.