Echiura

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Spoonworm, common name for any of a small phylum of unsegmented worms with an internal body cavity and characterized by a stout body, or trunk, and a long flexible, spoon-shaped proboscis, or feeding organ. They are found on the ocean bottom from the shallow intertidal zone to depths of 10,000 m (32,808 ft.)

Mature spoonworms, also called echiurans, are sedentary bottom feeders that burrow in mud, sand, or debris. A few live in rock crevices or enclosures such as abandoned sand dollar shells. Most spoonworms are dull in color, but a few are green, red, or transparent. With the proboscis retracted, they range in length from 1 to 20 cm (0.4 to 7.9 in). In most species, the trunk of the spoonworm lies buried in the muck or debris on the ocean bottom while the proboscis extends outward to grope for food. The touch- and taste-sensitive proboscis is flexible and may extend up to 2 m (6.56 ft). It has a groove lined with cilia (tiny hairs) along its length. Glands on the proboscis secrete sticky mucus that picks up the debris on which the spoonworm feeds. The cilia in the groove wave to transport mucus and debris to the mouth. The skin of the spoonworm is rich with sensory cells and mucous glands. Three layers of muscle line the interior of the body cavity, or coelom. The spoonworm moves by exerting pressure with these muscles on the coelomic fluid.

Spoonworms have a three-part digestive system consisting of a foregut, a stomach, and a long, coiled midgut, in which most digestion takes place. The circulatory system is very simple, consisting of two vessels that run the length of the body cavity and does not include a heart. A ventral nerve cord along the underside of the body cavity and a ring of nervous tissue near the head comprise most of the spoonworm's nervous system. Spoonworms breathe by diffusion of oxygen through the skin. All spoonworms have a pair of funnel organs, pocket-like sacs with openings on the underside of the animal.

The sexes are separate in spoonworms and fertilization occurs externally in most species. Gametes, or sex cells, are produced in sex organs near the ventral nerve cord. Sperm or eggs usually exit the body through the funnel organs. In some species the eggs are retained and the organs function as a uterus. The larvae, classified as trochophores, are similar to those of polychaetes, or segmented worms, and in most species of spoonworm, they float freely in the ocean for several months before settling as adults. In some spoonworms, males and females are very different and sex is determined by an unusual mechanism. In the green bonellid, if the larva settles away from others of its species, it becomes a female, up to 8 cm (3.15 in) long. If, however, it lands on or near the proboscis of a female of the same species, the larva will become a male, due to a hormone produced by the female's proboscis. The tiny males are only 2 to 3 mm (0.08 to 0.12 in) long and live on the female's body or internally in one of her funnel organs.

Scientific classification: The spoonworms comprise the phylum Echiura. The green bonellid is Bonellia viridis. The innkeeper is Urechis caupo.

Types of Invertebrates

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Annelida -> Segmented worms with a muscular body wall used for burrowing. External hairs called setae aid in traction during burrowing. An internal coelom is divided into compartments by walls known as septum. The digestive system stretches from the mouth to the anus, differentiated into regions, each with a different function. Reproduction is sexual. – Examples: Lugworms, earthworms, leeches

Pogonophora -> Deep-sea worms that live in chitinous tubes attached to the ocean floor. Their long, slender body has a beard of tentacles at the head end. There is no mouth or digestive system and these animals absorb all nourishment through the body surface. They reproduce sexually. – Examples: Bead worms

Vestimentifera -> Giant deep-sea worms that live in chitinous tubes attached to the ocean floor. They derive nutrition using a specialized organ called a trophosome to digest sulfide-oxidizing bacteria. – Examples: Llamellibrachs

Sipuncula -> Marine worms with a saclike body and a long proboscis. These worms withdraw their narrow head into the fatter posterior portion. The head end bears tentacles used in burrowing and gathering food particles. Lacking a cardiovascular system, they use internal fluid to transfer oxygen and food to body tissues. They reproduce sexually and some have a larval form. -> Peanut worms

Echiura -> Plump marine worms that take shelter in sand burrows or rock crevices. They use a mucous net or a scoop-shaped proboscis to capture food particles. Reproduction is sexual. – Examples: Spoon worms

Priapulida -> Cucumber-shaped, marine worms with spiny heads. During movement the barrel-shaped proboscis withdraws into the trunk of the worm. Reproduction is sexual. – Examples: Priapulus

Tardigrada -> Microscopic animals with four pairs of stubby legs that live in marine and freshwater sediments and on the surface of mosses and lichens. They use sharp stylets protruding from their mouths to suck food from plant cells. These animals have a remarkable ability to withstand extreme dryness and low temperatures. Reproduction is sexual. – Examples: Water bears

Pentastomida or Linguatulida -> Parasitic worms that live in the lungs of snakes, crocodiles, and some mammals and birds, feeding on blood and tissue. The head bears four leglike claws and a snoutlike mouth. The body is covered by a cuticle that is molted during larval development. They reproduce sexually. – Examples: Tongue worms

Phoronida -> Cylindrical, marine worms that live in a chitinous tube embedded in sand or attached to rocks, shells, or other objects in shallow water. Protruding out of the tube is the animal's lophophore, a structure of ciliated tentacles arranged in a horseshoe shape, that is used for feeding. They have a U-shaped digestive tract. Adults are sedentary and larvae are free-swimming. Reproduction is asexual in at least one species, but most species reproduce sexually. – Examples: Horseshoe worms

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