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.

Types of Invertebrates

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Kinorhyncha or Echinodera -> Tiny worms with spiny bodies. An outer protective cuticle is segmented and articulated. Found in the muddy bottoms of coastal waters, they feed on microorganisms and organic particles by means of a sucking pharynx. Reproduction is sexual. Examples: Echinoderes, Condyloderes

Nematoda -> Commonly known as roundworms, these animals are one of the most diverse and geographically widespread invertebrate phyla. Free-living roundworms inhabit freshwater and marine habitats, as well as soil. Parasitic roundworms prey on both plants and animals, causing widespread agricultural damage and disease. Roundworms have long, cylindrical bodies with a mouth surrounded by lips and sensory papillae or bristles. Fluid in the body cavity distributes nutrients and oxygen—roundworms do not have special respiratory or circulatory systems. Roundworms prey on other invertebrates as well as diatoms, algae, and fungi. They reproduce sexually and larvae undergo at least four molts before reaching their adult size and shape. – Examples: Ascarids, vinegar eels, cyst nematodes, heartworms, hookworms

Nematomorpha -> Long, threadlike worms found in soil or freshwater, commonly known as horsehair worms. They have no distinct head. Larvae are parasitic on terrestrial arthropods, usually insects. Adults do not feed but depend entirely on nutrients obtained during the parasitic larval stage. Reproduction is sexual. – Examples: Nectoneme, gordian worms

Acanthocephala -> Worms characterized by the presence of retractable spiny hooks that attach to the intestinal walls of aquatic and terrestrial vertebrates. Lacking a digestive tract, these worms absorb nutrients through their body wall. Reproduction is sexual and the life cycle includes larval forms that are parasites of crustaceans or insects. – Examples: Spiny-headed worms

Gnathostomulida -> Elongated, ciliated worms that live in marine sand. These animals have a mouth structure with a combed plate and toothed jaws. Bacteria and fungi are scraped into the mouth by the comb and passed into the gut by snapping movements of the jaws. Hermaphroditic, these animals reproduce sexually. – Examples: Jaw worms

Mollusca -> Diverse animals found in water and on land. Most mollusks have a hard shell that protects a soft body, although in some mollusks the hard shell is missing or hardly visible. A feeding organ called a radula contains rows of teeth used to scrape food into the mouth. Enzymes in salivary glands partially digest food before it reaches the intestines. Reproduction is sexual and some mollusks have a larval form. – Examples: Chitons, oysters, snails, clams, squid

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