Phylum Nematomorpha
Phylum
Nematomorpha
The popular name for Nematomorpha (nem´a-to-mor´fa) (Gr. nema, nematos, thread, + morphe, form) is “horsehair worms,” based on an old superstition that the worms arise from horsehairs that happen to fall into water, and they look something like hairs from a horse’s tail. They were long included with nematodes, with which they share the structure of the cuticle, presence of epidermal cords, longitudinal muscles only, and pattern of nervous system. However, since the early larval form of some species has a striking resemblance to the Priapulida, it is impossible to say to what group the nematomorphs are most closely related.
About 250 species of horsehair worms have been named. Worldwide in distribution, they are free living as adults and parasitic in arthropods as juveniles. Adults do not feed but will live almost anywhere in wet to moist surroundings if oxygen is adequate. Some juveniles, such as Gordius (named for an ancient king who tied an intricate knot), a cosmopolitan genus, are believed to encyst on vegetation that may later serve as food for a grasshopper or other arthropod. In the marine form Nectonema (Gr. nektos, swimming, + nema, thread), juveniles occur in hermit crabs and other crabs.
Form and Function
Horsehair worms are extremely long and slender, with a cylindrical body. Their length ranges from 10 to 70 cm, but their diameter is only 0.3 to 2.5 mm. Their anterior end is usually rounded, and their posterior end is rounded or has two or three caudal lobes (Figure 15-18).
The body wall is much like that of nematodes: a secreted cuticle, a hypodermis, and musculature of longitudinal muscles only. Ventral, or dorsal and ventral, but not lateral, hypodermal cords are present. In most nematomorphs the ventral nerve cord is connected to the ventral hypodermal cord by a nervous lamella.
The digestive system is vestigial. The pharynx is a solid cord of cells, and the intestine does not open to the cloaca. Larval forms absorb food from their arthropod hosts through their body wall, and adults apparently live on stored nutrients.
Circulatory, respiratory, and excretory systems are lacking. There are a nerve ring around the pharynx and a midventral nerve cord.
Juveniles do not emerge from their arthropod host unless water is nearby. Adults are often seen wriggling slowly about in ponds or streams, with males being more active than females. Each sex has a pair of gonads and a pair of gonoducts that empty into the cloaca. Females discharge eggs into the water in long strings. Juveniles hatch from the eggs and somehow gain entry into the arthropod host. After several months in the hemocoel of the host, the matured worm emerges into the water. Curiously, if the host is a terrestrial insect, the parasite stimulates the insect by an unknown mechanism to seek water.
The popular name for Nematomorpha (nem´a-to-mor´fa) (Gr. nema, nematos, thread, + morphe, form) is “horsehair worms,” based on an old superstition that the worms arise from horsehairs that happen to fall into water, and they look something like hairs from a horse’s tail. They were long included with nematodes, with which they share the structure of the cuticle, presence of epidermal cords, longitudinal muscles only, and pattern of nervous system. However, since the early larval form of some species has a striking resemblance to the Priapulida, it is impossible to say to what group the nematomorphs are most closely related.
About 250 species of horsehair worms have been named. Worldwide in distribution, they are free living as adults and parasitic in arthropods as juveniles. Adults do not feed but will live almost anywhere in wet to moist surroundings if oxygen is adequate. Some juveniles, such as Gordius (named for an ancient king who tied an intricate knot), a cosmopolitan genus, are believed to encyst on vegetation that may later serve as food for a grasshopper or other arthropod. In the marine form Nectonema (Gr. nektos, swimming, + nema, thread), juveniles occur in hermit crabs and other crabs.
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| Figure 15-18 Structure of Paragordius, a nematomorph. A, Longitudinal section through the anterior end. B, Transverse section. C, Posterior end of male and female worms. Nematomorphs, or “horsehair worms,” are very long and very thin. Their pharynx is usually a solid cord of cells and is nonfunctional. Paragordius, whose pharynx opens through to the intestine, is unusual in this respect and also in the possession of a photosensory organ (“eye”). |
Horsehair worms are extremely long and slender, with a cylindrical body. Their length ranges from 10 to 70 cm, but their diameter is only 0.3 to 2.5 mm. Their anterior end is usually rounded, and their posterior end is rounded or has two or three caudal lobes (Figure 15-18).
The body wall is much like that of nematodes: a secreted cuticle, a hypodermis, and musculature of longitudinal muscles only. Ventral, or dorsal and ventral, but not lateral, hypodermal cords are present. In most nematomorphs the ventral nerve cord is connected to the ventral hypodermal cord by a nervous lamella.
The digestive system is vestigial. The pharynx is a solid cord of cells, and the intestine does not open to the cloaca. Larval forms absorb food from their arthropod hosts through their body wall, and adults apparently live on stored nutrients.
Circulatory, respiratory, and excretory systems are lacking. There are a nerve ring around the pharynx and a midventral nerve cord.
Juveniles do not emerge from their arthropod host unless water is nearby. Adults are often seen wriggling slowly about in ponds or streams, with males being more active than females. Each sex has a pair of gonads and a pair of gonoducts that empty into the cloaca. Females discharge eggs into the water in long strings. Juveniles hatch from the eggs and somehow gain entry into the arthropod host. After several months in the hemocoel of the host, the matured worm emerges into the water. Curiously, if the host is a terrestrial insect, the parasite stimulates the insect by an unknown mechanism to seek water.
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