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NEMATODA
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NEMATODA , in See also:zoology, a See also:group of See also:worms. The name Nematoda (Gr. vi a, See also:thread, and eTSos, See also:form) was first introduced by Rudolphi, but the group had been previously recognized as distinct by Zeder under the name Ascarides. They are now bymany systematists See also:united with the See also:Acanthocephala and the See also:Nematomorpha to form the group Nemathelminthes. The Nematoda possess an elongated and thread-like form (see fig. I), varying in length from a few lines up to several feet. The See also:body is covered externally by a chitinous cuticle which is a product of the subjacent epidermic layer in which no See also:cell limits can be detected though nuclei are scattered through it. The cuticle is frequently prolonged into spines and papillae, which are especially See also:developed at the anterior end of the body. The mouth opens at one extremity of the body and the anus at or near the other. Beneath the epidermis is a See also:longitudinal layer of muscle-See also:fibres which are separated into four distinct See also:groups by the dorsal, ventral and lateral areas; these are occupied by a continuation of the epidermic layer; in the lateral areas run two thin-walled tubes with clear contents, which unite in the anterior See also:part of the body and open by a See also:pore situated on the ventral See also:surface usually about a See also:quarter or a third of the body length from the anterior end. These vessels are the nitrogenous excretory See also:organs. The body-cavity is largely occupied by processes from the large muscle cells of the skin. These processes stretch across the body cavity to be inserted in the dorsal and ventral See also:middle lines. The body-cavity also" contains the so-called phagocytic organs. These consist of enormous cells with nuclei so large as to be in some cases just visible to the naked See also:eye. These cells are disposed in pairs, though the members of each pair are not always at the same level. The number of cells is not large (some 2 to 8), and as a See also:rule they See also:lie along the lateral lines. In some See also:species (Ascaris decipiens) the See also:giant cell is replaced by an irregular See also:mass of See also:protoplasm containing a number of small nuclei. Such a plasmodium bears, on its periphery, groups of rounded projections of protoplasm termed end-organs. Similarly the giant cells are produced at their periphery into a number of branching processes which See also:bear similar end-organs on their surface and in some cases terminate in them. These end-organs are the active agents in taking up See also:foreign granules, or bacteria, which may have found their way into the fluid of the body-cavity. From the shape and position of the phagocytic organs it is obvious that they form admirable strainers through which the fluid of the body-cavity filters (See also:figs. 2, 3). The alimentary See also:tract consists of a straight See also:tube See also:running from the mouth to the anus without any convolutions; it is separable into three divisions:. (I) a See also:muscular See also:oesophagus, which is often provided with cuticular See also:teeth; (2) a cellular intes- After Galeb, See also:Arch. de Zool. Exp., 1878. oe, Oesophagus. bd, Enlargement of the oesophagus, armed with chitinous teeth. i, See also:Intestine. s, Opening of segmental tubes (placed by See also:mistake on the dorsal instead of the ventral surface). te, Testes. cd, Vas deferens. tine; and (3) a See also:short terminal rectum sp, See also:Cloaca. surrounded by muscular fibres. Neither pa, Papillae. here nor elsewhere are See also:cilia found at any See also:period of development. A See also:nervous See also:system has been shown to exist in many species, and consists of a perioesophageal See also:ring giving off usually six nerves which run forwards and backwards along the lateral and median lines; these are connected by numerous See also:fine, circular threads in the sub-cuticle. Some of the See also:free-living forms possess eye specks. The sexes are distinct (with the exception of a few forms that are hermaphrodite), and the male is always smaller than the See also:female. The generative organs consist of one or two tubes, in the upper portion of which the ova or spermatozoa are developed, the See also:lower portion serving as an oviduct or vas deferens; the female generative organs open at the middle of the body, the male See also:close to the posterior extremity into the terminal portion of the alimentary See also:canal; from this cloaca a diverticulum is given off in which are developed one to three chitinous spicules that subserve the See also:function of copulation. The spermatozoa differ from those of other animals in having the form of cells which sometimes perform amoeboid movements. Most remarkable sexual conditions are found to occur in the free-living genera Rhabditis and 1, Mouth. 2, Anterior end of alimentary canal. 3, Posterior end of alimentary canal. 4, Ovary. 5, 6 and 7, Anterior middle and I, See also:Nucleus of giant-cell. posterior pairs of phago- 2, One of the processes and endcytic organs. organs of the same. Diplogaster. While some of the species are bisexual, others are protandrous, self-fertilizing hermaphrodites. In cultures of the latter there occur very rare supplemental See also:males which appear in no sense degenerate but as See also:fit for See also:reproduction as the males of the bisexual species. Though possessing a See also:complete copulatory apparatus and producing large quantities of spermatozoa, they have lost their sexual See also:instinct and See also:play no part in the See also:economy of the species. These " psychically decadent " individuals appear to represent the entire male See also:sex of a bisexual species, and become unnecessary owing to the grafting of hermaphrodit- ism on the female sex. Mode of See also:Life and Metamorphoses.—While the See also:majority of theNematodes are parasites, there are many that are never at any period of their life parasitic. These free-living forms are found everywhere—in See also:salt and fresh See also:water, in See also:damp See also:earth and See also:moss, and among decaying substances; they are always See also:minute in See also:size, and like many other lower forms of life, are capable of retaining their vitality for a See also:long period even when dried, which accounts for their wide See also:distribution; this See also:faculty is also possessed by certain of the parasitic Nematodes, especially by those which See also:lead a free existence during a part of their life-See also:cycle. The free-living differ from the majority of the parasitic forms in under-going no See also:metamorphosis; they also possess certain structural peculiarities which led See also:Bastian (Trans. Linn. See also:Soc., 1865) to See also:separate them into a distinct See also:family, the Anguillulidae. It is impossible, however, to draw a strict See also:line of demarcation between the free and parasitic species, since—(1) many of the so-called free Nematoda live in the slime of molluscs (Villot), and are therefore really parasitic; (2) while certain species belonging to the free-living genus Anguillula are normally parasitic (e.g. A. tritici, which lives encysted in ears of See also:wheat), other species occasionally adopt the parasitic mode of existence, and become encysted in slugs, snails, &c.; (3) it has been experimentally proved that many normally parasitic genera are capable of leading a free existence;' (4) transitional forms exist which are free at one period of their life and parasitic at another. The parasitic Nematodes include by far the greatest number of the known genera; they are found in nearly all the orders of the See also:animal See also:kingdom, but more especially among the Vertebrate, and of these the See also:Mammalia are infested by a greater variety than any of the other groups. Some two dozen distinct species have been described as occurring in See also:man. The Nematode parasites of the Invertebrate are usually immature forms which attain their full development in the body of some vertebrate; but there are a number of species which in the sexually adult See also:condition are See also:peculiar to the Invertebrate? The Nematoda contain about as many parasitic species as all the other groups of See also:internal parasites taken together; they are found in almost all the organs of the body, and by their presence, especially when encysted in the tissues and during their See also:migration from one part of the body to another, give rise to various pathological conditions. Although some attain their full development in the body of a single See also:host—in this respect differing from all other Entozoa—the majority do not become sexually mature until after their transference from an " intermediate " to a " definitive " host. This migration is usually accompanied by a more or less complete metamorphosis, which is, however, not so conspicuous as in most other parasites, e.g. the Tremotoda. In some cases (many species of Ascaris) the metamorphosis is reduced to a See also:simple See also:process of growth. The parasitic and free-living Nematodes are connected by transitional forms which are free at one See also:stage of their existence and parasitic at another; they may be divided into two classes — those that are parasitic in the larval See also:state but free when adult, and those that are free in the larval state but parasitic when adult. (I) To the first class belong the so-called " hairworm," Mermis, not to be confused with the See also:Gordian worms.2 The adult forms of M. nigrescens live in damp earth and may be seen after storms or See also:early in the See also:morning crawling up the stalks of See also:plants, a fact which causes See also:people to talk about showers of worms. The eggs are laid on ' Ercolani successfully cultivated Oxyuris curvula, Strongylus armatus and other species in damp earth; the free See also:generation was found to differ from the parasitic by its small size, and by the See also:females being ovoviviparous instead of oviparous. To this phenomenon he gave the name of dimorphobiosis. 2 The genera Ascaris, Filaria, Trichosoma are found throughout the Vertebrate; Cucullanus (in the adult condition) only in fishes and See also:Amphibia; Ankylostoma, Trichocephalus, Trichina and Pseudalius live only in the Mammalia, the last-mentioned genus being confined to the See also:order See also:Cetacea; Strongylus and Physaloptera are peculiar to mammals, birds and See also:reptiles, while Dispharagus, Syngamus and Hystrichis are confined to birds. Mermis (in the larval state) is confined to the Invertebrate and Sphaerularia to bees. Oxyuris, though chiefly parasitic in the Mammalia, occurs also in reptiles, Amphibia and one or two See also:insects. Dacnitis and lehthyonema are only found in fishes. 2 See NEMATOMORPHA.. the ground and the See also:young larvae make their way into grasshoppers, in whose bodies they pass most of their larval life. (2),To the second class belong Ankylostoma, Strongylus and many species of Ascaris; the embryo on leaving the See also:egg lives free in water or damp earth, and resembles very closely the free-living genus Rhabditis. After a longer or shorter period it enters the alimentary canal of its proper host with drinking-water, or it bores through the skin and reaches the See also:blood-vessels, and is so conveyed through the body, in which it becomes sexually mature. Rhabditis nigrovenosa has a developmental See also:history which is entirely anomalous, passing through two sexual generations which regularly alternate. The See also:worm inhabits the See also:lung of the See also:frog and See also:toad, and is hermaphrodite (See also:Schneider) or parthenogenetic (Leuckart) ; the embryos hatched from the eggs find their way through the lungs into the alimentary canal and thence to the exterior; in a few days they develop into a sexual larva, called a Rhabditiform larva, in which the sexes are distinct; the eggs remain within the uterus, and the young when hatched break through its walls and live free in the perivisceral cavity of the See also:mother, devouring the organs of the body until only the See also:outer cuticle is See also:left; this eventually breaks and sets free the young, which are without teeth, and have therefore lost the typical Rhabditis form. They live for some See also:time in water or mud, occasionally entering the bodies of water snails, but undergo no See also:change until they reach the lung of a frog, when the cycle begins anew. Although several species belonging to the second class occasionally enter the bodies of water snails and other animals before reaching their definitive host, they undergo no alteration of form in this intermediate host; the See also:case is different, however, in Filaria medinensis and other forms, in which a free larval is followed by a parasitic existence in two distinct hosts, all the changes being accompanied by a metamorphosis. Filaria medinensis—the See also:Guinea worm—is parasitic in the subcutaneous connective See also:tissue of man (occasionally also in the See also:horse). It is chiefly found in the tropical parts of See also:Asia and See also:Africa, but has also been met with in See also:South Carolina and several of the See also:West See also:Indian islands. The adult worm in the female sometimes reaches a length of 6 ft. The males have only recently been discovered. The female is viviparous, and the young, which, unlike the See also:parent, are provided with a long tail, live free in water; it was formerly believed from the frequency with which the legs and feet were attacked by this See also:parasite that the embryo entered the skin directly from the water, but it has been shown by Fedschenko, and confirmed by See also:Manson, Leiper and others, that the larva bores its way into the Jody of a Cyclops and there undergoes further development. It is probable that the parasite is then transferred to the alimentary canal of man by means of drinking-water, and thence makes its way to the subcutaneous connective tissue. The Nematoda which are parasitic during their whole life may similarly be divided into two classes—those which undergo their development in a single host, and those which undergo their development in the bodies of two distinct hosts. (1) In the former class the eggs are extruded with the faeces, and the young become fully formed within the egg, and when accidentally swallowed by their host are liberated by the solvent See also:action of the gastric juice and complete their development. This simple type of life-history has been experimentally proved by Leuckart to be characteristic of Trichocephalus affinis, Oxyuris ambigua and other species. (2) The life-history of 011ulanus tricuspis is an example of the second class. 011ulanus tricuspis is found in the adult state in the alimentary canal of the See also:cat; the young worms are hatched in the alimentary canal, and often wander into the body of their host and become encysted in the lungs, See also:liver and other organs; during the encystment the worm degenerates and loses all trace of structure. This wandering appears to be accidental, and to have nothing to do with the further See also:evolution of the animal which takes See also:place in those embryos which are voided with the excrement. Leuckart proved experimentally that these young forms become encysted in the muscles of mice, and the cycle is completed after the See also:mouse is de- voured by a cat. The well-known Trichinella spiralis (fig. 4) has a life-history closely resembl- See also:ing that of 011ulanus. The adult worm, which is of extremely minute size, the male being only bth and the female * of an See also:inch in length inhabits the alimentary canal of man and many other carnivorous mammalia; the young See also:bore their way into the tissues and become encysted in the muscles—within the muscle-bundles according to Leuckart, but in the connective tissue between them according to Chatin and others. The co-existence of the asexual encysted form and the sexually mature adult in the same host,' exceptionally found in 011ulanus and other Nematodes, is the rule in Trichinella; many of the embryos, however, are extruded with the faeces, and complete the life cycle by reaching the alimentary canal of rats and See also:swine which frequently devour human ordure361 Swine become infested with Trichinella in this way and also by eating the dead bodies of rats, and the parasite is conveyed to the body of man along with the flesh of " trichinized " swine. Importance in See also:Pathology.--Among See also:recent advances having medical import in our knowledge of the Nematodes, the See also:chief are those dealing with the parasites of the blood. F. bancrofti is known to live in the lymphatic glands, and its embryos Microfilaria sanguinis hominis nocturna, passing by the thoracic duct, reach the blood-vessels and circulate in the blood. Manson showed in 1881 that the larvae (Microfilariae) were not at all times See also:present in the blood, but that their See also:appearance had a certain periodicity, and the larvae of F. bancrofti. Microfilaria nocturna swarmed in the blood at See also:night-time and disappeared from the peripheral circulation during the See also:day, hiding away in the large vessels' at the See also:base of the lungs and of the See also:heart. Ten years later Manson discovered a second species, Filaria perstans, whose larvae live in the blood. They, however, show no periodicity, and are found continuously both by day and by night; and their larval forms are termed Microfilaria perstans. The adult stages are found in the sub-peritoneal connective tissue. A third form, Microfilaria diurna, is found in the larval stage in blood, but only in the daytime. ' The adult stage of this form is the Filaria See also:lea found in the subcutaneous tissues of the limbs. The presence of these parasites seems at times to have little effect on the host, and men in whose system it is calculated there are some 40-50 million larvae have shown no signs of disease. In other cases very serious disorders of the lymphatic system are brought about, of which the most marked is perhaps See also:Elephantiasis. Manson and See also:Bancroft suggested that the second host of the parasite is the See also:mosquito or See also:gnat, and for a long time it was thought that they were conveyed to man by the mosquito dying after laying her eggs in water, the larval nematodes escaping from her body and being swallowed by man. It is now held that the parasite enters the blood of man through the piercing mouth-parts at the time of biting. When first sucked up by the See also:insect from an infected man it passes into its See also:stomach, and thence makes its way into the thoracic muscles, and there for some time it grows. Next the larvae make their way into the connective tissue in the See also:pro-See also:thorax, and ultimately bore a channel into the base of the piercing apparatus and come to See also:rest between the hypopharynx and the labium. Usually two are found in this position lying See also:side by side; it would be interesting to know if these are male and female. From their position in the See also:proboscis the larvae can easily enter the blood of man the next time the mosquito bites (See also:Low, Brit. Med. Journ., See also:June 1900; See also: During the See also:act of biting the labium is See also:bent back, and as the piercing stylets enter the skin of the sufferer this bending becomes more and more acute. Grassi and Noe think that if the cavity of the labium be full of the larval nematodes this bending will burst the tissue, and through the See also:rent the larvae will See also:escape and make their way into the body of the host. Besides Anopheles, two species of Culex, C. penicillaris and C. pipiens, are also accused of transmitting the larvae. A paper by Noe (Atti Acc. Lincei, ix., 1900) seems to prove beyond doubt that the larvae of F. immitis are transmitted in the manner indicated. The adult worm is chiefly found in the heart of the See also:dog, and usually in the right side, which may be so packed with the worms as seriously to interfere with the circulation (fig. 5). The females produce thousands of larvae, which circulate in the blood, and show a certain periodicity in their appearance, being much more numerous in the blood at night than during the day. Importance as Pests.—Agriculturists now pay increased See also:attention to the nematodes that destroy their crops. A See also:good example of a fairly typical case is afforded by Heterodera schachtii, which attacks beetroot and causes See also:great loss to the See also:Continental See also:sugar manufacturers. The young larvae, nourished by the yolk fibres. (After Leuckart.) 300 embryos, and that See also:half of these are females, the number of descendants would be, after six generations, some 22,781 milliards (A. Strubell, Bibl. Zool., 1888-1889). Other species which have been recorded in the United Kingdom are Tylenchus devastatrix (See also:Kuhn), on oats, See also:rye and See also:clover roots; T. tritici, causing the 362 which remains over from the egg and by the remains of the mother which they have taken into their alimentary canal, make their way through the earth, and ultimately coming across the See also:root of a See also:beet, begin to bore into it. This they do by means of a spine which can be protruded from the mouth. Once within the root, they absorb the cell See also:sap of the parenchyma and begin to swell until their body projects from the surface of the root in A, View of the heart of a dog infested with Filaria immitis See also:Leidy; the right ventricle and base of the pulmonary artery have been opened: a, aorta; b, pulmonary artery; c, vena cava; d, right ventricle; e, appendix of left See also:auricle; f, appendix of right auricle. B, Female F. immitis, removed from the heart to show its length. the form of a tubercle (fig. 6). The reproductive organs do not begin to appear until the larva has twice cast its skin. After this a marked sexual dimorphism sets in. The female, hitherto indistinguishable from the male, continues to swell until she attains the outlines of a See also:lemon. Doing this she bursts the epidermis of the rootlet, and her body projects into the surround- ing earth. The male has a different life-history (fig. 7). After
the second larval
See also:moult, he passes
through a passive stage
comparable to the
pupa-See also:stadium of an
-b insect, and during this
stage, which occurs
inside the root, the
reproductive organs
are perfected. The
male next casts his
cuticle, and by means
of his spine bores
through the tissues of
the root and escapes
into the earth. Here
he seeks a female,
pairs, and soon after-
wards See also:dies. The eggs
of the female give
rise to embryos with-
in the body of the
mother; her other organs undergo a retrogressive change
and serve as See also:food for the young, until the body-See also:wall only
of the mother remains as a See also: c, Spine. d, Muscle of spine. e, Gland. f, Oesophagus. g, Bulb. h, See also:Nerve-ring. i, Excretory pore j, Oesophagus. k, Testis. 1, Intestine. m, Muscles moving spicule. n, Spicule. See also:ear-See also:cockle of wheat; Cephalobus rigidus (Schn.), on oats; Heterodera radicicola (Greef), on the roots of tomatoes, cucumbers, potatoes, turnips, See also:peach-trees, vines and See also:lettuce, and many other plants. See N. Nassonov, Arch. Mikr. Anat. (1900); Arch. parasit. (1898); Rabot, Lab. See also:Warsaw (1898); Zool. Anz. (1898); L. Jagerskiold, Centrbl. Bakter. (1898): J. Spengel, Zool. Anz. (1897) ; H. Ehlers, Arch. Naturg. (1899); O. See also:Hamann, See also:Die Nemathelminthen (1895). (F. E. Additional information and CommentsThere are no comments yet for this article.
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