Photo: A spoon worm (Bonellia) in Western Australia, with its forked tongue fully extended. (Photo by L. Newman & A. Flowers. Reproduced by permission.)
Evolution and systematics
As the echiurans' body has no large hard parts, fossils of these animals are rare. There are two fossils for this group: a fossil from Illinois, United States, dated from the Late Carboniferous and another fossil from Namibia, dated from the Late Cambrian.
A free-swimming trochophore larva is present in echiurans, sipunculans, mollusks, and annelidans, which suggests a phyllogenetic relationship. Echiurans and annelids have many features in common. The most important difference between them is the absence of segmentation in Echiura. Some authors consider echiurans to be properly placed within the phylum Annelida, though other studies have shown that echiurans and pogonophorans have a close affinity and both may be closer to mollusks than to annelidans or sipunculans.
The phylum Echiura encompasses about 160 species and 40 genera, divided into two classes: Echiuridea, with three orders and four families (Echiuridae, Bonellidae, Ikedidae, and Urechidae), and Sactosomatidea, with a single family, Sacto-somatidae, with one species, S. vitreum.
Echiurans, also known as spoon worms, have a body divided in two distinct regions: a sausage-shaped saccular, non-segmented trunk and a ribbon-like proboscis at the anterior end. The length of the trunk may range from 0.39 in (1 cm) up to >19.6 in (>50 cm) and may be gray, dark green, reddish brown, rose, or red. It may be thick or thin, smoothed or roughened by glandular or sensory papillae. Internally, layers of muscles are responsible for peristaltic movements of the trunk. A pair of chitinous golden-brown chaetae usually occurs ventrally on the anterior part of the trunk. Some echiu-rans have one or two rings of chaetae around the anus.
The proboscis may be short or long, scoop- or ribbon-like, and flattened or fleshy and spatulate. It is generally white, rose, green, or brown. The distal end may be truncate or bifid. It is muscular, mobile, and highly extensible and contractile. It is able to extend 10 times its body length and can reach 3.2-6.5 ft (1-2 m). The ventral surface of the proboscis is ciliated, which helps in the feeding process. The mouth is located ventrally at the base of the proboscis and the anus is at the posterior extremity of the trunk.
Echiurans are mainly marine, but some species live in brackish waters. The majority of spoon-worms are found in intertidal and shallow waters, but there are also species living at depths of 32,800 ft (10,000 m).
Echiurans usually live in a U-shaped burrow with both ends of the burrow open. They are found mainly in soft ben-thic substrata such as sand, mud, or rubble, occupying burrows excavated by themselves or by other animals. Some species live in rock galleries excavated by boring invertebrates, whereas others live in empty shells, sand-dollar tests, coral or rock crevices, inside dead corals, or under stones. In general, some commensals are present inside the burrow, including polychaetes, crabs, mollusks, and fishes. The burrow provides
sand or mud. The movements by the peristalsis forces water through the tube, permitting the animal to obtain a supply of oxygen. In general, the burrow is kept clean and free from debris and fecal matter.
The food of echiurans consists of dead organic matter and microorganisms that live on the substratum. Echiurans may be detritus feeders. They extend the proboscis out of the burrow onto the surface of the sediment. The tip of the ventrally ciliated surface collects particles, and glands produce mucus to adhere to these particles. Movements of the cilia conduct particles and mucus to the mouth. The proboscis is then extended in a new direction, and the procedure is repeated. A few species are filter feeders. The innkeeper worm, Urechis caupo, constructs a mucous net placed near the opening of the burrow. Peristaltic movements of the trunk draw water through the burrow, and particles and small organisms are trapped in the net. Ultimately, the worm eats both the net and the food.
Echiurans reproduce strictly by sexual means. Sexual dimorphism is pronounced only in the family Bonellidae, in which the male is much smaller than the female. Sexes are separate, and sperm and eggs are usually liberated at the same time in seawater where fertilization occurs, except in Bonel-lidae, in which individuals undergo internal fertilization.
Free-swimming and feeding trochophore larvae develop 22 hours to four days following fertilization. The larvae may drift in the plankton for up to three months, and during metamorphosis, it increases in length. The larva settles on the substratum and begins life as an adult.
a protected, ventilated home, and remains of food discarded Conservation status by the spoon-worm may be eaten by the commensals. No species of Echiura are listed by the IUCN.
Echiurans are slow but not sedentary, and animals without a proboscis can swim. One of the most important movements is the peristalsis of the trunk, which allows the animal to move slowly over the surface and construct burrows in the
Some species of echiurans are commonly used as laboratory animals for physiological, embryological, and biochemical studies. The substance bonellin has been studied because of its antibiotic properties.
1. Female green bonellia (Bonellia viridis); 2. Male green bonellia (Bonellia viridis); 3. Innkeeper worm (Urechis caupo). (Illustration by Bruce Worden)
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