ASCHELMINTH

phylum name Aschelminthes, or Nemathelminthes, phylum of wormlike invertebrates, mostly of microscopic size. The phylum includes five diverse classes: Nematoda (or Nemata), Rotifera, Gastrotricha, Kinorhyncha (or Echinodera), and Nematomorpha. The American zoologist Libbie H. Hyman, in her classic textbooks on the invertebrates, originally included Priapulida as a class of the aschelminths, but Priapulida are usually not now included. Aschelminths have in common a body cavity, the pseudocoel, that arises in the embryo in a way different from that found in more advanced animals and that has no epithelial liningi.e., it is not a true coelom. Priapulids possess such an epithelial lining and are therefore coelomates. Aschelminths are bilaterally symmetrical, have a tough external covering, the cuticle, and, except for the kinorhynchs, lack segmentation. phylum name Aschelminthes, phylum of invertebrate animals distinguished by their possession of a pseudocoel (a space between the body wall and the gut), their exhibition of bilateral symmetry (in which the body is divided into mirror-image halves), and their lack of segmentation. Comprising approximately 17,000 known species, aschelminths are divided into five diverse classes: Nematoda, Rotifera, Gastrotricha, Kinorhyncha (or Echinodera), and Nematomorpha (or Gordiacea). Some zoologists argue that because of the extreme diversity of the classes, each should be raised to the level of a phylum. Nematoda (eelworms, pinworms, threadworms, and roundworms) is the largest of the five classes, numbering approximately 13,000 species. Featuring both free-living and parasitic forms, the nematodes occur in salt water, freshwater, sand, soil, plants, animals, and human beings, and they range in size from 0.1 to 2 mm (0.004 to 0.08 inch). The primary mode of nematode locomotion involves a sinuous movement brought about by alternating contractions of dorsal and ventral longitudinal body-wall muscles, although some species are able to remain stationary by means of an adhesive cement formed by the caudal glands. The parasitic order Filariida is of medical importance to humans. Rotifera (wheel animalcules) numbers approximately 1,800 species, which are found primarily in freshwater environments, although a few forms are known to inhabit salt water as well. Microscopic in size, rotifers generally feed on organic materials like bacteria, protozoans, and detritus, although a few species are carnivorous. Locomotion in nonsedentary forms is accomplished by means of beating cilia. Gastrotricha numbers approximately 1,800 known species, which are found primarily in bodies of freshwater, although there are a few saltwater species as well. They are microscopic, wormlike animals that generally feed (like the rotifers) on small organic materials. Gastrotrichs move by gliding across a surface using cilia. Kinorhyncha numbers approximately 100 known species, which are always found in marine environments. They are microscopic, wormlike animals that possess a retractable head and 13 or 14 superficial segments. Like rotifers and gastrotrichs, they feed on small organic materials. Kinorhynchs move by extending the head, then with hooked spines, anchoring it to a surface while they pull the rest of the body forward. Nematomorpha (hairworms) numbers approximately 300 species, which are always found in freshwater environments, except for the saltwater genus Nectonema. They are long, thin, wormlike animals that can reach lengths of up to 80 cm (30 inches). Before reaching maturity (when they become free-living adults), they are parasitic in such arthropods as crabs, millipedes, and centipedes. Priapulids, which are sometimes considered to be a separate class (Priapulida), are most probably a distinct phylum. Priapulids are superficially segmented, carnivorous animals that are found in marine environments, sometimes buried in ocean mud at depths of up to 500 m (1,600 feet). See also priapulid. Most aschelminths are wormlike, although a few species of rotifers are virtually spherical. All five classes, except for the kinorhynchs, have tails or taillike structures that aid in such functions as locomotion, anchoring, and mating. Aschelminth heads are generally indistinct, although kinorhynchs have spherical heads that are extendable and retractable. Rotifer mouths are surrounded by a crown of cilia, while some adult species of nematomorphs have no mouths at all. Setae (bristles), scales, or spines are usually found on the body surfaces of the majority of aschelminths. Internally, aschelminths are composed of three tubes enclosed within each other: the epidermis and cuticle (inner and outer layers of the body wall), the muscles, and the pseudocoel. The cuticle may bear hooks and bristles in nematodes, external segments in kinorhynchs, and thick plates in rotifers. Musculature also varies from class to class, with nematodes and nematomorphs possessing only longitudinal muscles, and rotifers, gastrotrichs, and kinorhynchs generally possessing longitudinal and circular transverse muscles. The digestive system comprises the pseudocoel, and in most aschelminths it runs the entire length of the body. Depending on the species, aschelminths may be either male, female, or hermaphroditic, with eggs fertilized inside the female and larval development outside. Male nematomorphs deposit sperm near the female cloaca (a reproductive and excretory duct), male nematodes release sperm into the female's vulva, and male rotifers inject sperm directly through the female's body wall. Nematomorphs, kinorhynchs, and nematodes exhibit periodic shedding of the cuticle (molting) during larval development. Nematomorphs first hatch into free-living larva, then they enter the body of some arthropod and undergo the parasitic phase of their life, and finally, after a period of weeks or months, they leave the host, molt once more, and become lethargic, free-living adults. Additional reading Two comprehensive classical works on the aschelminths are L.H. Hyman, The Invertebrates, vol. 3, Acanthocephala, Aschelminthes, and Entoprocta, the Pseudocoelomate Bilateria (1951); and Pierre P. Grass (ed.), Trait de zoologie: anatomie, systmatique, biologie, vol. 4, fascicle 2, Nmathelminthes (nmatodes), and fascicle 3, Nmathelminthes (nmatodes, gordiacs), rotifres, gastrotriches, kinorhynques (1965). More recent accounts include Paul A. Meglitsch, Invertebrate Zoology, 2nd ed. (1972); and Vicki Pearse et al., Living Invertebrates (1987), especially ch. 12 and 13. Papers on various aspects of rotifers are collected in the published proceedings of the International Rotifer Symposium; four meetings had been held by 1987.There is a much larger literature on the nematodes than other aschelminths because of their importance to humans. Introductory works include Neil A. Croll and Bernard E. Matthews, Biology of Nematodes (1977); Armand Maggenti, General Nematology (1981); Warwick L. Nicholas, The Biology of Free-Living Nematodes, 2nd ed. (1984); and George O. Poinar, Jr., The Natural History of Nematodes (1983). Parasitic forms are discussed in William R. Nickle (ed.), Plant and Insect Nematodes (1984); Gerald D. Schmidt and Larry S. Roberts, Foundations of Parasitology, 3rd ed. (1985); and Norman D. Levine, Nematode Parasites of Domestic Animals and Man (1980). Nematodes in biologic research are the subject of Bert M. Zuckerman (ed.), Nematodes as Biological Models, 2 vol. (1980). Warwick L. Nicholas