Phoronida

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Phoronids, common name for a wormlike marine invertebrate animal, a kind of tube worm. There are about 12 species of phoronids. They are sedentary animals that can be as long as 38 cm (15 in). Most species secrete a protective tube in the mud or sand bottoms of shallow seas.

The phoronids are regarded as closely related to the bryozoans and brachiopods because they possess a lophophore, a horseshoe-shaped structure carrying ciliated tentacles (see Tentaculata). The tentacles serve to catch food materials suspended in the water, and the cilia move mucus-entrapped food to the mouth, located at the center of the lophophore. The digestive tract is U-shaped, so the anus is located near the "head" of the animal but outside the lophophore.

The phoronids possess a well-developed circulatory system. Some species are hermaphroditic, in which one individual may have both male and female organs. Fertilization generally occurs outside the body; however, in some species, the eggs develop at the base of the lophophore tentacles.

Scientific classification: Phoronids are members of the phylum Phoronida, of the kingdom Animalia.

Pentastomida

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Tongueworm is any of a group of parasitic, wormlike animals found in the respiratory system of vertebrates, especially reptiles. Tongueworms cling to the lining of the nasal passageways and lungs where they feed on the blood of the host. Also called pentastomids, tongueworms occur worldwide, although most species are tropical.

The body of the tongueworm is highly modified for parasitism, that is, obtaining nourishment from the body of another, usually larger host organism (see Parasite). Adults of most species are 2 to 13 cm (0.8 to 5.1 in) long. Their mouth is jawless and sometimes projects outward as a trunklike proboscis. The mouth is flanked by two pairs of small, fingerlike projections equipped with claws that are used to attach to the host. The name pentastomid, meaning five-mouthed, derives from the fact that, in some species, these four grasping organs resemble the proboscis and look like additional mouths. The single, true mouth leads to a long, straight gut, the front of which forms a muscular pharynx for sucking blood. Frontal glands near the mouth of the tongueworm produce substances that break down the host tissue or prevent the host's blood from clotting. Like most internal parasites, tongueworms have no organs of excretion, gas exchange (breathing), or circulation. The tongueworm's blood flows freely throughout the body cavity. The tongueworm's nervous system consists of a frontal ganglion, or primitive brain, and a ventral nerve cord that runs along the bottom of the body cavity.

The sexes of tongueworms are separate, and the females are larger than the males. Fertilization is internal and occurs within the primary host. The larva has two or three pairs of walking legs, each with two claws. The larva may be autoinfective, that is, it remains in the primary host. If not autoinfective, the larva may form a protective capsule, called a cyst, which either moves through the digestive tract to be deposited with the host's feces or moves passively out of the host's nose or mouth.

Tongueworm eggs may pass out of the host before they hatch. If an intermediate host animal accidentally swallows the cyst or egg, the infective larva emerges inside the new host. It bores through the gut wall and completes its development into the next infective stage. If the intermediate host is eaten by a predator, the predator may acquire the pre-adult tongueworm. The juvenile tongueworm then crawls up the esophagus to the respiratory system and implants itself in the lungs or nasal passageways to complete the cycle. The larvae of one species of tongueworm that infests reindeer can infest the next generation of hosts by boring through the placenta. Examples of typical intermediate hosts and their corresponding primary hosts are cockroaches and lizards, fish and crocodiles, and rabbits and dogs.

Scientific classification: Tongueworms make up the phylum Pentastomida. Their relationship to other animals is not clear; they resemble both arthropods and annelids. One theory on the origin of this group is that they are actually highly modified crustaceans.

Onychophora

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Velvetworm, any of a group of soft-bodied, many-legged, worm-like animals known for their velvety cuticle, or outer covering, and their resemblance to both arthropods (insects, spiders, and crustaceans) and annelids (segmented or true worms). They live in moist tropical and temperate habitats south of the tropic of Cancer. Velvetworms are also known as onychophorans or peripatuses, after the most familiar genus. About 80 species of velvetworms are known.

Mature velvetworms range from 1.5 to 15 cm (0.6 to 5.9 in) in length and may be blue, orange, green, or black in color. Their thin, flexible cuticle, or outer skin, bears many small, scaly, wartlike tubercles and sensory hairs, which produce a velvety appearance. Velvetworms have 14 to 43 pairs of stubby, claw-tipped legs called lobopods. The lobopods contain no muscles. Velvetworms control their body and limb movements with hydrostatic pressure changes within their body cavity. Velvetworms use these pressure changes to lift and lower their legs in waves, as their body creeps along with slug-like motion.

When conditions are too dry or too wet, velvetworms hide in burrows or protected niches until conditions improve. They are nocturnal, appearing only at night, and prey on other small invertebrates. The velvetworm's jaws hold the prey while it is being eaten. Salivary glands produce enzymes that partly digest tissues so the meal can be sucked up. When disturbed or threatened, velvetworms shoot a sticky, quick-hardening slime from two oral papillae, wartlike bumps on either side of the mouth. They can shoot the slime up to 50 cm (19.7 in). The slime is produced by slime glands originating at the base of the oral papillae and may also be used to immobilize prey.

The velvetworm has a pair of fleshy, ringed antennae on its head and a pair of eyes near the bases of the antennae, each with a lens and retina. Blood is circulated by a tube-shaped dorsal heart (located near the top of the body cavity) and flows freely within the body cavity around the internal organs; it carries no oxygen. Gases are exchanged through a system of minute tracheae, or breathing tubes, which open into pits located throughout the cuticle. The pits cannot be closed and are a major source of water loss for the moisture-dependent velvetworm. The velvetworm takes in water by drinking and by absorption through special ballooning sacs on the lobopods. Wastes are excreted from paired sacs called coelomoducts or nephridia, which are present on most body segments and which lead to pores on the legs. The slime glands and female sexual ducts are modified coelomoducts. The brain gives rise to a pair of ventral nerve cords, which run along the bottom of the body cavity and are connected by ladderlike cross nerves.

The sexes are separate in velvetworms; males are generally smaller and may have fewer legs than females. In the species where sexual behavior is known, fertilization occurs through copulation.

Scientific classification: Velvetworms make up the phylum Onychophora. The most familiar species are in the genus Peripatus.

Nematoda

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Roundworm, also nematode, common name for any of a phylum of unsegmented terrestrial, freshwater, or marine worms. Roundworms are almost worldwide in distribution and are abundant in the surface layers of soils. Many of them are economically and medically harmful, living as parasites in plants and animals, including humans. Roundworm infections are common and frequently go unnoticed, but several species cause serious diseases.

Roundworms are cylindrical, tapering animals with simple bodies consisting of an interior gut and a muscular outer wall, separated by a fluid-filled cavity called a pseudocoel (see Animal: Coelom). The outer wall secretes an elastic cuticle that is molted four times during the animal's lifetime. Species range in size from microscopic to about 10 cm (about 4 in) long. Most species have separate sexes, but a few are hermaphroditic; fertilization is internal. The young roundworms, which resemble the adults, develop without metamorphosis.

Although numerous roundworms are free-living, the parasitic forms are of greatest economic interest. One important group, the ascaroid nematodes, includes the threadworms and the common worm of puppies. Another contains the eelworms, which produce root knot of cotton, and forms that produce earcockle of wheat. Other, medically significant forms of roundworm include the various genera known as hookworm; the filaria, which cause elephantiasis; the trichina worm, the cause of trichinosis; and the whipworm, which infests the human intestine.

Scientific classification: Roundworms make up the phylum Nematoda. Threadworms belong to the genus Ascaris. The common worm of puppies is classified as Toxocara canis. Eelworms belong to the genus Heterodera, the forms that produce earcockle of wheat belong to the genus Tylenchus, and the trichina worm belongs to the genus Trichinella. The human whipworm is classified as Trichuris trichiura.

Nematomorpha

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Horsehair Worm, member of a group of long, slender, unsegmented worms that resemble horsehair. The name of their phylum, Nematomorpha, means “threadlike.” Long ago, people thought that these worms were actually hairs from a horse's tail that had come alive. The immature worms, or larvae, are all parasites, feeding within the bodies of leeches and arthropods such as insects and crustaceans. The adults are free-living—that is, they do not depend on a host.

Horsehair worms are 1 to 3 mm (0.04 to 0.12 in) in diameter and up to 1 m (3.3 ft) long. There are two classes of horsehair worms. The first, known as nectonemes, live within the bodies of crabs and their relatives. The adults are planktonic, drifting in the currents of the open ocean. The body has two rows of bristles down each side, which aid in buoyancy. Nectonemes have only one gonad, or sex organ. The second class of horsehair worms is made up of the gordian worms, so named because they appear to tie themselves in knots, much like the complex Gordian knot of Greek mythology. The larvae parasitize leeches and arthropods such as grasshoppers, crickets, beetles, millipedes, and centipedes. The adults occur worldwide in freshwater or moist terrestrial habitats. Gordian worms possess a pair of gonads.

The body of the adult horsehair worm is covered by a relatively thick outer cuticle, or skinlike covering. Underlying the cuticle is a layer of longitudinal muscle running the length of the body. The horsehair worm has no circular muscle, and thus its movement is limited to lashing and curling. The digestive system is simple, even in the free-living adults. There are no excretory, circulatory, or respiratory organs. The larvae probably take up nutrients directly through the body wall during the parasitic phase. Feeding has not been observed in the adults, and most have no mouth, but they grow considerably after they leave the host at the end of the larval stage. It is therefore likely that they do feed by some means. The nervous system consists of a nerve bundle at the head end and a nerve cord running down length of the body. Around the head of some species is a ring of pigmented tissue that may be light-sensitive, but horsehair worms have no eyes. The outer layer of the cuticle is often covered with bumps, some of which may detect touch and some of which produce a lubricant.

Scientific classification: Horsehair worms make up the phylum Nematomorpha. The nectonemes make up the class Nectonematoidea, which has only one genus, Nectonema. The gordians make up the class Gordioidea.