worm, common name for various unrelated invertebrate animals with soft, often long and slender bodies. Members of the phylum Platyhelminthes, or the flatworms, are the most primitive; they are generally small and flat-bodied and include the free-living planarians (of the class Turbellaria) as well as the parasitic flukes (class Trematoda) and tapeworms (class Cestoda). The nemertines, or ribbon worms (phylum Nemertinea), are often colorful marine carnivores with an extensible proboscis. The smallest species are only a fraction of an inch (less than 2.5 cm) long, while giants of the group range up to 90 ft (27 m) and are the longest of all invertebrates. Pseuodcoelomate worms include those in the phyla Rotifera, Gastrotricha, Kinorhyncha, Nematoda, and Nematomorpha. Of these, the largest phylum is the nematodes, which are probably the most numerous multicellular animals. Also called roundworms and threadworms, the nematodes include widespread free-living species as well as parasites, such as the hookworm. Other parasitic nematodes include Filaria, the cause of filariasis, which may result in elephantiasis; Trichinella, the cause of trichinosis; Ascaris, an intestinal parasite of humans, horses, and pigs; the pinworm, a parasite common in children; the Guinea worm, Dracunculus medinensis, which is ingested as a larva in water and slowly emerges when full grown (up to 3 ft/91 cm) through a painful sore in the skin, but as a result of an eradication program is now found only in Chad, Ethiopia, and South Sudan; and various other species that are agricultural pests. Like the nematodes, the hairworms, or horsehair worms, are unsegmented, but they are grouped separately in the phylum Nematomorpha. The larvae are parasitic, first in the bodies of aquatic insects and then within grasshoppers or beetles. The adult is about 6 in. (15 cm) long and covered with brown chitin, giving it a stiff appearance; since the worms were frequently found in watering troughs, superstition had it that they developed from horsehairs. The annelid worms (phylum Annelida) have segmented bodies, distinct heads, digestive tubes, circulatory systems, and brains. Appendages on each segment are used for walking or swimming. They include the earthworm, of the class Oligochaeta, the leech (class Hirudinea), and the marine annelids of the class Polychaeta. The sea mouse, the clam worm, and the feather duster worm belong to the latter group. The shipworm is a type of clam. The larvae of many insects are popularly called worms. Moth and butterfly larvae can be distinguished from adult animals called worms by the presence of several pairs of fleshy appendages at the rear end of the body (see caterpillar). However, other insect larvae are completely legless, while still others are equipped with six pairs of legs, as in adult insects (see larva). Insect larvae known as worms include the armyworm, bagworm, cutworm, and inchworm.
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