1911 Encyclopædia Britannica/Nematoda
NEMATODA, in zoology, a group of worms. The name Nematoda (Gr. νῆμα, thread, and εἶδος, form) was first introduced by Rudolphi, but the group had been previously recognized as distinct by Zeder under the name Ascarides. They are now by many systematists united with the Acanthocephala and the Nematomorpha to form the group Nemathelminthes.
The Nematoda possess an elongated and thread-like form (see fig. 1), varying in length from a few lines up to several feet. The body is covered externally by a chitinous cuticle which is a product of the subjacent epidermic layer in which no cell limits can be detected though nuclei are scattered through it. The cuticle is frequently prolonged into spines and papillae, which are especially 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 longitudinal layer of muscle-fibres which are separated into four distinct 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 part of the body and open by a pore situated on the ventral surface usually about a quarter or a third of the body length from the anterior end. These vessels are the nitrogenous excretory 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 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 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 rule they lie along the lateral lines. In some species (Ascaris decipiens) the giant cell is replaced by an irregular mass of 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 bear similar end-organs on their surface and in some cases terminate in them. These end-organs are the active agents in taking up 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 (figs. 2, 3).
The alimentary tract consists of a straight tube running from the mouth to the anus without any convolutions; it is separable into three divisions: (1) a muscular oesophagus, which is often provided with cuticular teeth; (2) a cellular intestine; and (3) a short terminal rectum surrounded by muscular fibres. Neither here nor elsewhere are cilia found at any period of development.
A nervous system has been shown to exist in many species, and consists of a perioesophageal ring giving off usually six nerves which run forwards and backwards along the lateral and median lines; these are connected by numerous fine, circular threads in the sub-cuticle. Some of the 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 female. The generative organs consist of one or two tubes, in the upper portion of which the ova or spermatozoa are developed, the lower portion serving as an oviduct or vas deferens; the female generative organs open at the middle of the body, the male close to the posterior extremity into the terminal portion of the alimentary canal; from this cloaca a diverticulum is given off in which are developed one to three chitinous spicules that subserve the 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 Diplogaster. While some of the species are bisexual, others are protandrous, self-fertilizing hermaphrodites. In cultures of the latter there occur very rare supplemental males which appear in no sense degenerate but as fit for reproduction as the males of the bisexual species. Though possessing a complete copulatory apparatus and producing large quantities of spermatozoa, they have lost their sexual instinct and play no part in the economy of the species. These “psychically decadent” individuals appear to represent the entire male sex of a bisexual species, and become unnecessary owing to the grafting of hermaphroditism on the female sex.
Mode of Life and Metamorphoses.—While the majority of the Nematodes are parasites, there are many that are never at any period of their life parasitic. These free-living forms are found everywhere—in salt and fresh water, in damp earth and moss, and among decaying substances; they are always minute in size, and like many other lower forms of life, are capable of retaining their vitality for a long period even when dried, which accounts for their wide distribution; this faculty is also possessed by certain of the parasitic Nematodes, especially by those which lead a free existence during a part of their life-cycle. The free living differ from the majority of the parasitic forms in undergoing no metamorphosis; they also possess certain structural peculiarities which led Bastian (Trans. Linn. Soc., 1865) to separate them into a distinct family, the Anguillulidae. It is impossible, however, to draw a strict 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 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 animal kingdom, but more especially among the Vertebrata, and of these the Mammalia are infested by a greater variety than any of the other groups. Some two dozen distinct species have been described as occurring in man. The Nematode parasites of the Invertebrata 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 condition are peculiar to the Invertebrata.
The Nematoda contain about as many parasitic species as all the other groups of 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 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 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 Trematoda. In some cases (many species of Ascaris) the metamorphosis is reduced to a simple process of growth. .
The parasitic and free-living Nematodes are connected by transitional forms which are free at one stage of their existence and parasitic at another; they may be divided into two classes—those that are parasitic in the larval state but free when adult, and those that are free in the larval state but parasitic when adult.
(1) To the first class belong the so-called “hairworm,” Mermis, not to be confused with the Gordian worms. The adult forms of M. nigrescens live in damp earth and may be seen after storms or early in the morning crawling up the stalks of plants, a fact which causes people to talk about showers of worms. The eggs are laid on the ground and the 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 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 blood vessels, and is so conveyed through the body, in which it becomes sexually mature. Rhabditis nigrovenosa has a developmental history which is entirely anomalous, passing through two sexual generations which regularly alternate. The worm inhabits the lung of the frog and toad, and is hermaphrodite (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 mother, devouring the organs of the body until only the outer cuticle is 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 time in water or mud, occasion all entering the bodies of water snails, but undergo no 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 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 Guinea worm—is parasitic in the subcutaneous connective tissue of man (occasionally also in the horse). It is chiefly found in the tropical parts of Asia and Africa, but has also been met with in South Carolina and several of the West 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 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 parasite that the embryo entered the skin directly from the water, but it has been shown by Fedschenko, and confirmed by Manson, Leiper and others, that the larva bores its way into the body 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 action of the
gastric juice and complete their development. This simple type of
life-history has been experimentally proved by Leuckart to be
Fig. 4.—Trichinella encysted among muscular fibres. (After Leuckart.) characteristic of Trichocephalus affinis, Oxyuris ambigua and other species. (2) The life-history of Ollulanus tricuspis is an example of the second class. Ollulanus tricuspis is found in the adult state in the alimentary canal of the 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, 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 evolution of the animal which takes 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 mouse is devoured by a cat. The well-known Trichinella spiralis (fig. 4) has a life-history closely resembling that of Ollulanus. The adult worm, which is of extremely minute size, the male being only 1th and the female 1th of an inch in length inhabits the alimentary canal of man and many other carnivorous mammalia; the young 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 Ollulanus 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 swine which frequently devour human ordure. 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 Pathology.—Among recent advances having medical import in our knowledge of the Nematodes, the 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 present in the blood, but that their appearance had a certain periodicity, and the larvae of F. bancrofti, Microfilaria nocturna swarmed in the blood at night-time and disappeared from the peripheral circulation during the day, hiding away in the large vessels at the base of the lungs and of the 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 loa 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 Elephantiasis. Manson and Bancroft suggested that the second host of the parasite is the mosquito or gnat, and for along 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 insect from an infected man it passes into its 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 pro-thorax, and ultimately bore a channel into the base of the piercing apparatus and come to rest between the hypopharynx and the labium. Usually two are found in this position lying side by side; it would be interesting to know if these are male and female. From their position in the proboscis the larvae can easily enter the blood of man the next time the mosquito bites (Low, Brit. Med. Journ., June 1900; James, ibid., Sept. 1900). Shortly after Low had published his results, Grassi and Noè issued a paper dealing with the larvae of F. immitis, which is spread by means of the mosquito Anopheles (Centrbl. Bakter. I. Abth. xxviii., 1900). The larvae of this parasite develop in the Malpighian tubules of the insect; at a certain stage they cast their cuticle and make their way into the space—part of the haemocoel—found in the labium. During the act of biting the labium is bent back, and as the piercing stylets enter the skin of the sufferer this bending becomes more and more acute. Grassi and Noè think that if the cavity of the labium be full of the larval nematodes this bending will burst the tissue, and through the rent the larvae will 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 Noè (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 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 attention to the nematodes that destroy their crops. A good example of a fairly typical case is afforded by Heterodera schachtii, which attacks beetroot and causes great loss to the Continental sugar manufacturers. The young larvae, nourished by the yolk 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 root of a 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 sap of the parenchyma and begin to swell until their body projects from the surface of the root in 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 lemon. Doing this she bursts the epidermis of the rootlet, and her body projects into the surrounding earth. The male has a different life-history (fig. 7). After the second larval moult, he passes through a passive stage comparable to the pupa-stadium of an 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 afterwards dies. The eggs of the female give rise to embryos within the body of the mother; her other organs undergo a retrogressive change and serve as food for the young, until the body-wall only of the mother remains as a brown capsule. From this the young escape and make their way through the earth to new roots. The whole life-history extends over a period of some 4-5 weeks (fig. 7), so that some 6-7 generations are born during the warmer months. If we assume that each female produces 300 embryos, and that 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 (Kuhn), on oats, rye and clover roots; T. tritici, causing the ear-cockle of wheat; Cephalobus rigidus (Schn.), on oats; Heterodera radicicola (Greef), on the roots of tomatoes, cucumbers, potatoes, turnips, peach-trees, vines and lettuce, and many other plants.
A, View of the heart of a dog infested with Filaria immitis 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 auricle; f, appendix of right auricle.
B, Female F. immitis, removed from the heart to show its length.
A, a, Female Heterodera schachtii Schmidt, breaking through the epidermis of a root; the head is still embedded in the parenchyma of the root.
B, a, larvae boring their way into a root; b, larva of the immobile kind surrounded by the old skin, living as an ectoparasite on the outside of the root. (From Strubell.)
|A,||Male Heterodera schachtii, greatly magnified.|
|d,||Muscle of spine.|
|m,||Muscles moving spicule.|
|B,||First motile larva.|
|C,||Second immovable parasitic larva casting its skin.|
|D,||A female with one half of the body-wall taken away to show the coiling generative organs.|
|E,||A male shortly before casting its larval skin.|
See N. Nassonov, Arch. Mikr. Anat. (1900); Arch. parasit. (1898); Rabot, Lab. Warsaw (1898); Zool. Anz. (1898); L. Jägerskiold, Centrbl. Bakter. (1898); J. Spengel, Zool. Anz. (1897); H. Ehlers, Arch. Naturg. (1899); O. Hamann, Die Nemathelminthen (1895). (F. E. B.; A. E. S.)
- Ercolani successfully cultivated Oxyuris curvula, Strongylus armatus and other species in damp earth; the free generation was found to differ from the parasitic by its small size, and by the females being ovoviviparous instead of oviparous. To this phenomenon he gave the name of dimorphobiosis.
- The genera Ascaris, Filaria, Trichosoma are found throughout the Vertebrata; Cucullanus (in the adult condition) only in fishes and Amphibia; Ankylostoma, Trichocephalus, Trichina and Pseudalius live only in the Mammalia, the last-mentioned genus being confined to the order Cetacea; Strongylus and Physaloptera are peculiar to mammals, birds and reptiles, while Dispharagus, Syngamus and Hystrichis are confined to birds. Mermis (in the larval state) is confined to the Invertebrata and Sphaerularia to bees. Oxyuris, though chiefly parasitic in the Mammalia, occurs also in reptiles, Amphibia and one or two insects. Dacnitis and Ichthyonema are only found in fishes.
- See Nematomorpha.