Gnathostomulida

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Gnathostomulid, group of microscopic marine worms that live between grains of sand in shallow ocean water. These invertebrates are able to live for long periods without oxygen (see Anaerobe) and are common in stagnant bottom muck and black sand. More than 80 species of gnathostomulids have been described.

Gnathostomulids resemble flatworms and were once classified in the same phylum. Like flatworms, gnathostomulids are bilaterally symmetrical (both sides of the body are identical along the midline) and have waving body cilia (tiny hairlike projections); they lack an anus and a coelom (internal body cavity) and are hermaphroditic (individuals have both male and female sex organs). Unlike flatworms, however, each epithelial (skin) cell in gnathostomulids has only one hairlike cilium. In addition there are significant differences in sperm structure between gnathostomulids and flatworms.

Gnathostomulids are generally 0.5 to 1.0 mm (0.02 to 0.04 in) in length with a slight constriction of the body separating the head from the trunk. They move their long transparent bodies with the propelling force of their rotating cilia. Unlike flatworms, they have the ability to reverse the direction of the ciliary beat. The contractions of three or four pairs of longitudinal muscle fibers also contribute to movement. Gnathostomulids have a mouth located on the underside of the body behind the head. They graze on bacteria and fungi using a pair of toothed jaws that grasp and push food into the intestinal sac.

Even though gnathostomulids possess both male and female sex organs, they cannot fertilize themselves. Eggs are fertilized when one gnathostomulid injects a packet of sperm into another individual’s body using a penislike organ that is equipped with a piercing tip in some species. A single large egg is released by breaking through the body wall and develops without larval stages into an adult. The parent worm regenerates rapidly after releasing the egg. Some gnathostomulids appear to alternate between a nonsexual feeding phase and a sexual phase during which they do not feed.

Scientific classification: Gnathostomulids make up the phylum Gnathostomulida, and are members of the superphylum Aschelminthes.

Gastrotricha

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Gastrotrich, group of microscopic wormlike animals that are common in aquatic environments worldwide. The gastrotrich lives in bottom sediments and on the surface of submerged vegetation and debris. Its head is divided into lobes and its body is generally flat, transparent, and unsegmented. Its back and sides are spiny, bristly, or scaly. The flattened bottom surface bears small hairs, or cilia, in characteristic patterns that are helpful in classifying species. Along the sides of the body are adhesive tubes that are used for clinging to vegetation.

Gastrotrichs range in length from 0.05 to 4.0 mm (0.002 to 0.16 in). Marine gastrotrichs, those living in the ocean, are abundant on the surfaces of corals and in shallow sandy areas, while freshwater species prefer standing water such as puddles, marshes, and wet bogs. They may be present in habitats where there is much decay and they can withstand low levels of dissolved oxygen for short periods. The gastrotrich feeds as it moves, ingesting bacteria, algae, small protozoans, and organic debris from the substrate. Waving cilia on the head create currents that push food particles to the mouth.

Marine gastrotrichs are hermaphroditic (having both male and female sex organs) and individuals alternately produce eggs and sperm. Most freshwater species are entirely female and reproduce through parthenogenesis (egg development without fertilization). A female will produce one to five very large eggs in its lifetime. The eggs are of two types: thin-walled eggs that develop as soon as they are laid, and thick-walled eggs that require exposure to harsh conditions such as drying, freezing, or high temperatures before they develop. Gastrotrichs have no larval stages. On emergence from the egg, the gastrotrich is already about 25 percent of its adult size. It grows rapidly and some species reach sexual maturity in only three days. The lifespans of gastrotrichs in nature are unknown, but individuals in laboratory cultures live 3 to 22 days. Gastrotrichs are important as food for many organisms in aquatic food webs.

Scientific classification: Gastrotrichs make up the phylum Gastrotricha, which contains two orders. Animals in the order Macrodasyida are strictly marine, and those in the order Chaetonotida are primarily freshwater.

Entoprocta

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Entoprocts, tiny, mosslike invertebrate animals found in shallow waters. Entoprocts have tentacles and oval-shaped bodies, and usually live in branching colonies. Entoprocts lack a true coelom, or body cavity. A slender stalk attaches the body mass, or calyx, to either an inanimate object or another animal. The digestive tract is U-shaped. On the uppermost surface of the calyx are tentacles made of cilia arranged in a circle around the mouth and anus. The movements of the hairlike cilia create currents that sweep tiny animals and suspended particles into the mouth.

Entoprocts may live singly but are more often found as colonial groups, with many individuals arising from a single animal through the asexual process of budding. When sexual reproduction occurs, large yolky eggs are brooded in a special cavity within the circle of tentacles. With the exception of one freshwater genus, the entoprocts are exclusively marine. Because of their small size they are difficult to observe without the aid of a microscope.

Scientific classification: Entoprocts make up the phylum Entoprocta.

Ectoprocta

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Moss Animals, common name for two phyla of small, simple aquatic animals that feed with a crown of tentacles called a lophophore and usually form attached, mossy colonies. The classification of these two phyla has varied to reflect changing opinions about the relationship of moss animals to other phyla. Authorities who think the two groups have a close common ancestor retain the phylum name Bryozoa for all moss animals and treat endoprocts and ectoprocts as classes. Others use the term Bryozoa only for ectoprocts, and still others think that ectoprocts are related to the Tentaculata and endoprocts to the Aschelminthes.

Endoprocts, which are marine except for one freshwater species, have a globular body that is mounted on a stalk. The lophophore surrounds both mouth and anus. The animals reproduce both sexually and asexually, often forming colonies of connected individuals by the latter process. In ectoprocts, which are primarily marine, the lophophore does not surround the anus. The colonies that ectoprocts form by asexual reproduction are of varied structure, and each member usually has a hard, protective coating.

Scientific classification: Endoprocts make up the division Endoprocta, or Entoprocta. Ectoprocts make up the phylum Ectoprocta, or Bryozoa.

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.