Echiuroid worms, or echiurans, commonly called spoon worms, are soft-bodied, unsegmented, marine animals of worldwide distribution. The approximately 125 species in the phylum Echiura occur mostly in the shallow intertidal zone of oceans. Most burrow or form tubes in sand or mud. Some live in discarded shells of sea urchins and sand dollars. Others inhabit cracks and crevices in rocks or coral fragments. Body length varies from a fraction of an inch to 20 in (50 cm) or more. There are two body divisions: the body proper, or trunk; and a proboscis, which is highly mobile and extensible, but not retractable into the trunk. The trunk may be smooth, or it may have rows of small papillae or tubercles, giving it a superficially segmented appearance. At its anterior end, close to the base of the proboscis, it bears a pair of curved or hooked chitinous processes called setae. (Presence of setae is a major characteristic of the phylum Annelida, in which the setae are more numerous.) In some species there are additional setae near the posterior end of the trunk. The body wall is muscular, and a spacious, fluid-filled cavity separates it from the gut. This cavity does not extend into the proboscis. The gut is much longer than the trunk, and parts of it are coiled. It begins at the mouth at the base of the proboscis, and terminates at the anus at the opposite end.
The proboscis may be short, broad, and spoon-shaped, as in the common genus Echiurus, or it may be much longer than the trunk, narrow, and divided into two branches at the tip as in Bonellia. Its edges are rolled over ventrally to form a trough, which is lined by cilia and mucus-secreting cells. It is used in feeding to collect organic particles from the sandy or muddy substrate and to transport it to the mouth. Tube-dwelling echiurans, for example Urechis, collect their food by filter-feeding. This worm secretes a mucus funnel that strains out particles from water pumped into the tube. From time to time the mucus is transported to the mouth and ingested, and a new funnel is formed.
Respiratory gas exchange in echiuroid worms occurs between the body fluid and sea water, usually across the body wall. But at least in some species, water is pumped in and out of the lower part of the gut through the anus, and gas exchange takes place across the wall of the gut. Although a circulatory system of closed vessels is present, the blood is colorless and serves mainly to transport nutrients. Excretion is performed by tubular structures called nephridia whose number varies widely among the species. One end of each nephridium is funnel-shaped and ciliated, and opens into the body cavity. The other end is narrow and opens to the outside by means of a minute pore. The nervous system is simple, without a brain or specialized sense organs. Sexes are separate. Each individual has a single gonad, testis, or ovary, which develops from the lining of the body cavity. Immature gametes are shed into the body cavity. Upon maturation they are transported to the outside through the nephridial tubes. Fertilization is external, and a planktonic "trochophore" larva (found also in the segmented worms, phylum Annelida, and a few other groups) develops.
Although in most echiurans males and females of a particular species look alike, an interesting example of sexual dimorphism and sex differentiation is presented by the European form Bonellia viridis. In this species, the male, which is ciliated and lacks a proboscis, is about 0.4-0.8 in (1-2 mm) long. In contrast, the female's trunk alone is 2.3-3.1 in (6-8 cm), with an even longer proboscis. The male lives in the female's pharynx or occasionally in her body cavity. The trochophore larva in this species develops into a female if it settles (at metamorphosis) some distance away from an existing female. On the other hand, if it settles close to a female, it develops into a male. It has been suggested that females produce and release into the surrounding water a "hormone" which has a masculinizing effect on the developing trochophore.