fig68Fig. 68.—Diagram to explain the descriptive formula used for Nematodes. (From Cobb.) 6, 7, 8, 10, 6 are the transverse measurements, while 7, 14, 28, 50, 88 are the corresponding longitudinal measurements. The formula in this case is714285088678106The unit of measurement is the one-hundredth part of the length of the worm. The measurements are therefore percentages of the length.The measurements are taken with the animal viewed in profile; the first is taken at the base of the oesophagus, the second at the nerve-ring, the third at the cardiac constriction, the fourth at the vulva in females and at the middle in males, the fifth at the anus.
Fig. 68.—Diagram to explain the descriptive formula used for Nematodes. (From Cobb.) 6, 7, 8, 10, 6 are the transverse measurements, while 7, 14, 28, 50, 88 are the corresponding longitudinal measurements. The formula in this case is714285088678106The unit of measurement is the one-hundredth part of the length of the worm. The measurements are therefore percentages of the length.The measurements are taken with the animal viewed in profile; the first is taken at the base of the oesophagus, the second at the nerve-ring, the third at the cardiac constriction, the fourth at the vulva in females and at the middle in males, the fifth at the anus.
Fig. 68.—Diagram to explain the descriptive formula used for Nematodes. (From Cobb.) 6, 7, 8, 10, 6 are the transverse measurements, while 7, 14, 28, 50, 88 are the corresponding longitudinal measurements. The formula in this case is
The unit of measurement is the one-hundredth part of the length of the worm. The measurements are therefore percentages of the length.
The measurements are taken with the animal viewed in profile; the first is taken at the base of the oesophagus, the second at the nerve-ring, the third at the cardiac constriction, the fourth at the vulva in females and at the middle in males, the fifth at the anus.
Taking everything into consideration, it has seemed advisable in the following systematic account of the Nematoda to abandon the larger groups, and to deal directly with the families. Claus distinguishes seven of these, and the diagnoses given at the head of each are mainly taken from hisGrundzüge der Zoologie.[175]
I. Family Ascaridae.
Body rather stout. A dorsal and two ventro-lateral lips, bearing papillae. Buccal cavity distinct, seldom provided with chitinous armature. The oesophagus often has two dilatations.The tail of the male is ventrally curved, and usually there are two horny spicules. The Ascaridae are found in the intestines of their respective hosts.
Genera:Ascaris,Heterakis,Oxyuris,Nematoxys,Oxysoma, and many others.
Von Linstow[176]enumerates over 250 species ofAscaris, of which it will only be possible to mention here one or two. They are all parasitic in Vertebrata.
A. lumbricoidesLinn. is one of the largest known Nematodes ♂ = 4-6 in., ♀ = 10-14 in.; Figs. 66 and 67). It is a common parasite in man, and has been found in the ox. It is now generally recognised as the same parasite which inhabits the pig, and which Dujardin regarded as specifically distinct, and namedA. suillae. In the latter host, however, it never attains the dimensions it does in man. It inhabits the upper and middle parts of the small intestine, and has been known to escape into the body-cavity and set up abscesses there, or to make its way into the stomach, and to be voided through the mouth. It is practically cosmopolitan in distribution, and is very common in Japan—Baely found it in twenty-one out of twenty-threepost-mortems—and in Tonquin and tropical Africa. Heller[177]states that no one is free from these worms in Finland, and they are common wherever there is a plentiful water supply, as in the marshy districts of Holland and Sweden. In Iceland alone they seem absent. When examined alive they give off an irritating vapour which seriously affects some observers, causing catarrhal symptoms, which in Bastian's case lasted six weeks. The usual number found in one host is small, one to six or eight, but cases are on record where many hundreds occurred in one person.
The details of the life-history of this form are not yet completely worked out. The eggs leave the body of the host with the excreta, and formerly it was thought they re-entered the alimentary canal in drinking-water, etc., and there developed into the adult without change of host. This view has been combated by Leuckart, who failed to rear the Nematodes by direct feeding, and it has been noticed that the youngest parasites found in theintestine are already 2 to 3 mm. long. Von Linstow has recently suggested that the larval stages may be hatched out in the body of the millipedeJulus guttulatus, whose habits might easily lead it to eat the eggs of the parasite in manured gardens, etc., and which is itself sometimes unconsciously eaten when hidden in fruit or vegetables. This would account for the frequent presence of the parasite in pigs, and also for the fact that in man it is commonest in children who are apt to eat windfalls, and in maniacs and people with perverted tastes.
A. megalocephala, which is found in the horse, ass, zebra, ox, etc., attains even greater dimensions than the foregoing. The male rarely exceeds 7 inches in length, but the female sometimes reaches 17 inches. They are found in the small intestine of their hosts. Cobbold[178]succeeded in rearing larvae which attained a high degree of organisation when the eggs were placed amongst moist horse-dung, and it seems probable that the larvae pass into the body of their hosts in drinking water; at any rate no intermediate host has yet been found, and Davaine, who fed cows, and Leuckart, who fed horses with the unhatched eggs, both failed to infect the animals they experimented on.A. mystax, which lives in cats, dogs, and other Carnivora, has also been found in man. It is provided with fin-like extensions on the side of its head (cf. Fig. 62), and varies much in size in different hosts. When first found in man it received the name ofA. alata. It becomes sexually mature in about three weeks.
One of the most remarkable cycles of development amongst the many curious life-histories met with amongst Nematodes, is that presented byRhabdonema(Ascaris)nigrovenosum. The free form of this, formerly known as a distinct species,Rhabditis nigrovenosa, lives in the excrement of frogs, and attains sexual maturity in a very short time. The sexes pair, and the fertilised ova give rise to embryos which hatch out within the body of the mother, and then begin to devour her internal organs. After the destruction of the mother, the embryos escape and live in water or slime, and sometimes burrow into water snails, but they undergo no change until swallowed by a frog. Then they make their way into its lungs and grow enormously, attaining a length of almost an inch. This form, parasitic in the frog, is a protandrous hermaphrodite, which first produces spermatozoa and afterwardsova; the latter are fertilised by the spermatozoa, and give rise to rhabditiform embryos, which escape by the alimentary canal and form the free-living sexual stage mentioned above. Thus in the life-history of this form we find an alternation of generation, a sexual free-living form alternating with a hermaphrodite parasitic form.
Of the enormous number of other species of the genus, only a very few can be mentioned.A. transfugaRud. inhabits bears;A. leptopteraRud., lions;A. feroxH. and Ehrbg., Hyracoidea;A. depressaRud., vultures;A. rubicundaSchn., pythons;A. sulcataRud., turtles;A. mucronataSchn., the cod and pike;A. incurvaRud., the sword-fish.
fig69Fig. 69.—A male and femaleOxyuris diesingiHam.in copula, × 60.a, Anus;b, oesophagus;c, bulb;d, testis;e, intestine;f, ovary. (From Galeb.[179])
Fig. 69.—A male and femaleOxyuris diesingiHam.in copula, × 60.a, Anus;b, oesophagus;c, bulb;d, testis;e, intestine;f, ovary. (From Galeb.[179])
Fig. 69.—A male and femaleOxyuris diesingiHam.in copula, × 60.a, Anus;b, oesophagus;c, bulb;d, testis;e, intestine;f, ovary. (From Galeb.[179])
Oxyurisis Meromyarian (see p.137), and is characterised by the long capillary tail of the female. It includes another human parasite,O. vermicularis, and it is one which it is difficult to get rid of. The female has the characteristic tail and is about 10 mm. long. The male is smaller. They are found in the caecum and rectum of man, and cause great irritation and sometimes serious functional disturbance. The eggs are laid in immense numbers but perish in water. If whilst still in the egg-shell the larvae are swallowed on fruit or raw vegetables, etc., they are set free in the stomach and small intestine by the action of the digestive secretions. The distribution of this parasite is universal. Besides numerous species that inhabit the alimentary canal of Vertebrates, such asO. ambiguaRud., found in hares and rabbits;O. curvulaRud., inthe caecum of horses;O. megatyphlonRud., in iguanas; several species inhabit the rectum of insects, such asO. blattae,O. diesingi,O. blatticola, found in the cockroach;O. spirotheca, andO. hydrophiliin the water beetleHydrophilus.[180]
The genusNematoxyshas the most complex arrangement of muscles of any Meromyarian, and forms a transition to the Polymyarian type. The whole body of both sexes is covered with numerous irregularly scattered papillae. The members of this genus have hitherto been found in snakes, Amphibia, and eels; there are but few species.
Oxysomais another small genus with but three species, found in the intestines of opossums, frogs, and turtles respectively.
II. Family Strongylidae.
Mouth surrounded by papillae; an armature of teeth or spines often present. The chitinous lining of the intestine projects into the interior as ridges. No oesophageal bulb. The male orifice at the posterior end of the body is surrounded by a bell-shaped bursa.
Genera:Eustrongylus,Strongylus,Dochmius,Sclerostomum,Cucullanus,Syngamus,Pseudalius,Ollulanus, and others.
The genusEustrongylusincludes two species,E. gigasRud. andE. tubifexNitsch. The former attains in the female the gigantic length of 860 mm., with a breadth of 7 mm. and a weight of over 40 grs.[181]The male is a quarter to a third as long as the female. This parasite inhabits the kidney capsules of carnivorous animals, especially of those that eat fish, such as dogs, seals, etc., and has occasionally been found in man, the horse, and the deer. It frequently destroys the substance of the kidney. The worms are red in colour. The eggs die when exposed to desiccation for a few days, but have been kept alive for fifteen months in water; it is believed by Schneider and Leuckart that they are eaten by fish, and that the larvae form theFilaria cysticafound in the peritoneal membrane of the fishesGalaxias scribaandSymbranchus laticaudatus, and that they pass into their final host, where they become sexually mature, by the latter eating raw fish.E. tubifexis found in aquatic birds,e.g.ducks, grebes, and divers, etc.
The genusStrongylusis easily recognised by its conspicuous genital bursa, strengthened by variously arranged ridges which are of specific value. There are numerous species, found in man and many other mammals, and also in birds and reptiles. Some species inhabit the intestine, others form aneurisms in the large blood-vessels, and cause considerable mortality amongst horses; others live in the tracheae and lungs of cattle and sheep, their presence often causing great loss to the farmer. No intermediate host has been satisfactorily demonstrated; the larvae live in damp earth, and it seems almost certain that they pass directly into their host with its food.
Dochmius(Ancylostomum)duodenalis, called by Neumann[182]Uncinaria duodenalis, is one of the most dangerous parasites that attack man. It lives in the duodenum and jejunum, and the fertilised eggs leave the body of its host with the excreta, and in damp earth develop into larvae in the course of a few days. These at first eat voraciously, but after undergoing several moults they cease to take food and pass into the resting stage. If now they are swallowed with drinking water, they come to rest in the small intestine of their host, and in a few weeks become sexually mature. They cause great harm by burrowing in the intestinal walls and destroying the capillaries. They are found by hundreds, and even thousands, in the same host, and produce profound anaemia, which is frequently fatal to miners, and was the cause of a great mortality amongst the workers in the St. Gothard Tunnel some fifteen years ago. This species is very widely spread over the face of the globe.Dochmius trigonocephalaRud. andD. stenocephalaproduce similar diseases in dogs and cats, andD. cernuaCrep. is found in sheep and goats.
The genusCucullanusexists in the adult form in the intestines of fishes, and more rarely of reptiles.C. elegansZed., which live in fresh-water fish,e.g.the perch, is viviparous; after birth the young pass into the water and make their way into the alimentary canal of the small crustaceanCyclops, and thence into its body-cavity. Here they undergo two moults, accompanied by certain changes in structure. If this second host be swallowed by a fish the parasites are set free, and develop generative organs.Ollulanus tricuspisLeuck., which in the adult state is found in the cat, chiefly in the intestine but also in the bronchi and other parts, gives rise to larvae which are of enormous size compared with the parent; these leave the body, and if eaten by a mouse encyst in its muscles, and if the mouse be devoured by a cat, they complete their life-cycle by becoming sexually mature.
The genusSyngamusinfests the trachea and bronchi of birds, more rarely of mammals. Thered-orforked-worm,Syngamus trachealisSieb., is common in poultry and game birds, and causes the disease known as gapes, which is especially common in young birds, and often gives rise to extensive loss. The peculiarity of this genus is that the male is permanently attached to the female, its genital bursa being so closely adherent to the opening of the oviduct that two specimens cannot be separated without tearing the tissues. The ova are not laid, but escape from the body with fully-formed embryos in them, by the decay or rupture of their parent's body. They hatch in damp earth or water in from one to six weeks according to the temperature. When swallowed by a fowl they develop into adults, which reproduce eggs in less than three weeks. No second host is needed, but the embryos remain alive in the alimentary canal of earthworms, and these doubtless to some extent serve to spread the disease.
fig70Fig. 70.—Syngamus trachealisSieb., natural size and magnified four diameters. The small ♂ is permanently attached to the female. (From Warburton.[183])
Fig. 70.—Syngamus trachealisSieb., natural size and magnified four diameters. The small ♂ is permanently attached to the female. (From Warburton.[183])
Fig. 70.—Syngamus trachealisSieb., natural size and magnified four diameters. The small ♂ is permanently attached to the female. (From Warburton.[183])
III. Family Trichotrachelidae.
This family is characterised by the anterior end of the body being produced into a long whip-like neck. The mouth is small and devoid of papillae. The oesophagus is very long, and it traverses a peculiar strand of cells.
Genera:Trichocephalus,Trichina,Trichosoma, and others.
Trichocephalus disparRud. (hominisGmel.) is common in man, and also occurs in some species of monkey. It does not live freely within the intestine, but buries its long whip-like anterior end in the mucous lining of the caecum or colon. The eggs pass out of the body of the host. The development of the embryo is slow, lasting many months; whilst still in the egg-shell the embryos are swallowed, and give rise to the sexually-mature parasite without the intervention of an intermediate host. They are by no means uncommon. Davaine calculated that about 50 per cent of the inhabitants of Paris were infested with them, but they give rise to little disturbance, and only very occasionally cause serious harm.T. affinisRud. infests sheep;T. crenatusRud. the pig;T. depressiusculusRud. the dog; andT. unguiculatusRud. the hare and rabbit.
fig71Fig. 71.—Trichocephalus disparRud., attached to part of the human colon. × 2.
Fig. 71.—Trichocephalus disparRud., attached to part of the human colon. × 2.
Fig. 71.—Trichocephalus disparRud., attached to part of the human colon. × 2.
The genusTrichosoma, with many species, is as a rule found in birds, but it occurs also in mammals, asT. plicaRud. in the bladder of the fox and wolf,T. felis catiin the bladder of the cat,T. aerophilumDuj. in the trachea of the fox and marten. The chief interest of this genus is that, at any rate inT. crassicaudaBel., which infests the rat, the dwarf males live two, three, or four at a time within the uterus of the female, a condition of things which recalls the similar arrangement found in the GephyreanBonellia.
Trichina spiralisis the cause of the well-known disease trichinosis, which appears in two forms, intestinal and muscular, according to the habitat of the parasite. The mature forms ofboth sexes are found in the intestine of man and many other mammals. They have been experimentally developed in birds, though in the latter the larval forms have never been observed. By keeping such cold-blooded animals as the salamander at a constant temperature, Goujon and Legros succeeded in infecting them, but the larvae perished as soon as the artificial heat was withdrawn. Muscular trichinosis is unknown in fishes, but the sexual form develops in their intestine.
The adult parasites of the intestine are scarcely visible to the naked eye; the females are 3 to 4 mm. long and more numerous than the males, which measure 1.4 to 1.6 mm. The eggs are very numerous, a single female containing at one time 1200, and probably producing ten times as many during her life. The embryos are hatched out within the uterus, and the larvae leave the body of the mother through the generative pore. The minute larvae bore through the intestinal walls of their host, and then, either burrowing in the tissues or swept along in the stream of blood or lymph, make their way all over the body, and come to rest most usually in the muscles, but occasionally in other parts. When the larva reaches its resting-place, it either pierces the sarcolemma and establishes itself within the substance of the muscle-fibre, or it comes to rest between and not in the fibres. Here its presence sets up the formation of a spindle-shaped cyst which usually contains but one larva, though any number up to seven have been found in one cyst. Within this the larva may remain dormant for years, the walls of the cyst gradually undergoing a fatty or calcareous degeneration. Almost any muscle may be affected; those most usually infested being the muscles of the diaphragm, of the shoulder-blade, and of the lumbar region; the larvae have also been found in the heart. The ends of the muscles near their points of attachment are always the most thoroughly infested.
fig72Fig. 72.—Trichina spiralisOwen, encysted in muscle.a, Calcareous deposit. Highly magnified. (From Leuckart.)
Fig. 72.—Trichina spiralisOwen, encysted in muscle.a, Calcareous deposit. Highly magnified. (From Leuckart.)
Fig. 72.—Trichina spiralisOwen, encysted in muscle.a, Calcareous deposit. Highly magnified. (From Leuckart.)
The number of the encapsuled larvae in one host is enormous. Leuckart counted between 12,000 and 15,000 in a gramme of muscle, which would give a total of thirty to forty million parasites in one host; other estimates place the total even higher.
When trichinised meat is eaten, unless it has been thoroughly cooked, the cysts are dissolved and the larvae are set free. Within three or four days they become sexually mature and their ova begin to segment. The males after a time leave the body with the excreta and perish, whilst the larvae of the new brood make their way into the tissues of the host.
Man usually acquires trichinosis by eating uncooked or improperly-cooked pork, and the disease is so widely spread and of such a serious nature that most civilised countries have adopted rigorous methods for the detection of trichinised meat. The pigs either acquire the disease by eating uncooked swine's flesh, which is frequently given them in the form of offal, or by devouring rats, which are very susceptible to the disease.
IV. Family Filariidae.
Mouth with two lips, or without lips. Six oral papillae often present, and sometimes a horny oral capsule. Four pre-anal pairs of papillae, and sometimes an unpaired one as well. Two unequal spicula or a single one.
Genera:Filaria,Ichthyonema,Hystrichis,Spiroptera,Dispharagus, and others.
The genusFilariais a very large one. LikeAscaris, it is confined to Vertebrates, but usually lives in the tissues of the body and not in the intestines.F.(Dracunculus)medinensisGmel., the guinea-worm, is well known as a human parasite in hot countries; it also occurs in the horse and dog. The female has an average length of 50 to 80 cm., but gigantic forms with a length of 4 metres have been described. The alimentary canal is degenerate. In adult females the body is completely occupied by a uterus crowded with eggs and embryos, which can only escape by the rupture of the mother's body, as the genital ducts have disappeared. Its original home is tropical Asia and Africa, but it has been introduced into South America with the negroes.
The female lives coiled up in the subcutaneous tissues, usually in those of the legs. Its presence gives rise to painful tumours. When these break the female protrudes, and may be withdrawn from the body by very carefully rolling it round a stick or pencil. This must be done very slowly, a few inches a day, as the rupture of the body sets free the contained embryos, and may result inthe death of the host. The embryos normally bore their way into the body of the fresh-waterCyclops, and are re-introduced into their Vertebrate hosts with the drinking-water. It is usually stated that the female alone is known, and that it is uncertain whether it is hermaphrodite or whether both sexes are present in theCyclops. Recently Dr. Charles[184]has described a specimen found in the mesentery of a human subject, from an orifice in the middle of whose body he was able to draw a much smaller specimen, and he thinks this may be the long-sought-for male.
fig73Fig. 73.—A, View of the heart of a dog infested withFilaria immitis[185]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, A femaleF. immitisremoved from the heart to show its length. Natural size.
Fig. 73.—A, View of the heart of a dog infested withFilaria immitis[185]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, A femaleF. immitisremoved from the heart to show its length. Natural size.
Fig. 73.—A, View of the heart of a dog infested withFilaria immitis[185]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, A femaleF. immitisremoved from the heart to show its length. Natural size.
Filaria immitisLeidy, the cruel worm, is common in dogs in China and the East generally. It is not unknown in America and Europe. It occurs in such large clusters in the right ventricle that it is difficult to see how the circulation can proceed. The intermediate host is unknown, but from the prevalence of thedisease in marshy country it is probably some aquatic animal. The larvae are said by Manson to disappear from the peripheral circulation of the dog during the day, but not to such a marked extent as do theF. sanguinis hominisLew., var.nocturnaMan. They were found by Galeb and Pourquier in the foetus of an infested bitch, a fact which establishes the transmission of such parasites through the placenta.
Filaria sanguinis hominis nocturna.—The female of this parasite has been described as living in the lymphatic glands of man. The embryos escape from it into the lymph, and thus reach the blood. According to Manson the intermediate host is the mosquito, in whose stomach the embryos undergo their larval changes. When the mosquito dies the larvae escape into the water, and then make their way into the alimentary canal of man, where they are believed to pair, and whence the female makes its way to the lymphatics. The presence of thisFilariacauses great functional disturbance. One of the most remarkable features of it is that the larvae, which are very numerous in the blood during the night, disappear during the day, and are not to be found. Recently Manson[186]has described two new varieties:F. san. hom. diurna, in which the conditions of things are reversed, the larvae being found by day and not by night; andF. san. hom. perstans, in which the larvae occur both by day and by night. The larvae are long-lived, and were found by Manson in the blood of a negro who had not been in Africa, where it is endemic, for six years. The same observer is inclined to associate the presence ofF. san. hom. perstanswith the fatal disease known as "sleeping sickness." He also suggests that the mature form of the varietydiurnais theF. loa, which is not uncommon in the eyes of negroes, and that its intermediate host may be one of the blood-sucking flies so common on the west coast of Africa.
The genusIchthyonemais confined to fishes. The male is very minute and the female partly degenerate. It has no anus and no external opening to its generative organs. The uterus fills up almost the whole of the body-cavity.I. sanguineumRud. is found encapsuled in the peritoneum of many fish.
HystrichisandDispharagusare confined to birds, where they occur in the oesophagus and stomach.Spiroptera reticulataCrep. occurs in horses, twisted in a spiral round tendons and muscles, forming tumours which require to be opened.
V. Family Mermithidae.
Nematodes without anus and with six mouth papillae. Two spicules in the males and three rows of numerous papillae.
Genera:Mermis,Bradynema,Atractonema,Allantonema,Sphaerularia, and others.
As a rule the Nematoda show but little trace of their parasitic mode of life, but in this family there is considerable degeneration, and in extreme cases the body of the female is reduced to a simple sac crowded with eggs. They are exclusively parasitic in insects. In some respects their structure shows a transition towardsNectonemaand the Gordiidae; especially is this the case in the structure of their ventral nerve-cord.
The sexual form ofMermis nigrescensDuj.[187]lives in damp earth, and after storms and in the early morning is sometimes found in such numbers crawling up the stalks of plants, as to give rise to the popular idea that there has been a shower of worms. The male is unknown; the female lays her eggs in the ground, and there they hatch out. It is not known exactly how the larvae make their way into the grasshoppers in whose body-cavity they live, but in an allied species,M. albicansv. Sieb., the larvae have been observed boring their way into small caterpillars through their skin, and it seems probable that the larvae ofM. nigrescensburrow in a similar way into young Orthoptera.
Bradynema rigidumLeuck.[188]is found in the adult stage living freely in the body-cavity of a small beetleAphodius fimetarius, one of the Scarabeidae, from two to three to as many as thirty being found in one host, which does not seem much injured by their presence. The parasite is without mouth, anus, or excretory pore. The eggs hatch out in the uterus of the mother, and the larvae are male and female; they make their way into the body-cavity of the host, and here they pass an unusually long time, five months, soaking in osmotically the nutriment contained in the blood of the insect. Eventually they burrow through the walls ofthe intestine, and leaving the body of their host through the anus, find their way to the earth. Here, according to zur Strassen, the females die without playing any part in the perpetuation of the species. The males, on the other hand, having developed spermatozoa whilst in the larval stage (paedogenesis), afterwards form ova, and are in fact protandrous hermaphrodites, and become the mature parasites of the beetle, though how they enter the body of the host is unknown.
fig74Fig. 74.—Allantonema mirabileLeuck. (From Leuckart.)A, Male Rhabditis stage, sexually mature, × 100;B, the mature female parasitic form, × 17, showing at the upper end part of the capsule richly supplied with the tracheae of the host, a beetle;C, female Rhabditis stage, sexually mature, × 100;D, the larva developed from the Rhabditis form, × 102.
Fig. 74.—Allantonema mirabileLeuck. (From Leuckart.)A, Male Rhabditis stage, sexually mature, × 100;B, the mature female parasitic form, × 17, showing at the upper end part of the capsule richly supplied with the tracheae of the host, a beetle;C, female Rhabditis stage, sexually mature, × 100;D, the larva developed from the Rhabditis form, × 102.
Fig. 74.—Allantonema mirabileLeuck. (From Leuckart.)A, Male Rhabditis stage, sexually mature, × 100;B, the mature female parasitic form, × 17, showing at the upper end part of the capsule richly supplied with the tracheae of the host, a beetle;C, female Rhabditis stage, sexually mature, × 100;D, the larva developed from the Rhabditis form, × 102.
The phenomenon presented by the hermaphroditism ofBradynemais, as far as we know, at present unique, as, though some other Nematodes are hermaphrodite, in their case the hermaphrodite form alternates with a bisexual generation. It is further interesting as showing a means by which hermaphroditism may arise, by the suppression of the females and the assumption of their functions by the male. In the case ofRhabdonema nigrovenosum, no females appear in the alternate generation.
fig75Fig. 75.—Atractonema gibbosumLeuck. (From Leuckart.) 1, Female with commencing prolapsus of the uterus and neighbouring parts, × 130; 2, a further stage, the female being now sexually mature, × 15; 3, a still older stage, with commencing degeneration of the body of the female, × 15.
Fig. 75.—Atractonema gibbosumLeuck. (From Leuckart.) 1, Female with commencing prolapsus of the uterus and neighbouring parts, × 130; 2, a further stage, the female being now sexually mature, × 15; 3, a still older stage, with commencing degeneration of the body of the female, × 15.
Fig. 75.—Atractonema gibbosumLeuck. (From Leuckart.) 1, Female with commencing prolapsus of the uterus and neighbouring parts, × 130; 2, a further stage, the female being now sexually mature, × 15; 3, a still older stage, with commencing degeneration of the body of the female, × 15.
A similar protandry exists in the parasitic forms ofAllantonema,[189]of which there are several species—A. mirabileinHylobius pini,A. sylvaticuminGeotrupes sylvatica,A. diplogasterinTomicus typographicus; but in their case the male and female forms which leave their host pair in the damp earth and give rise to larvae which make their way into the body of the beetle-grubs. Here they undergo very extensive retrogressive change. The body of the female, which becomes the shape of a thick sausage, is encapsuled and surrounded by a curious hypertrophied network of tracheae (Fig. 74). As is usually the case with the degenerate parasitic forms, there are practically no organs but the ovary, and this isembedded in a fatty parenchyma which fills all the space within the skin.
fig76Fig. 76.—Four stages in the life-history ofSphaerularia bombiDufour, ♀. (From Leuckart.)A, Beginning of the protrusion of the uterus (b), × 66;B, later stage, × 66;C, later stage, × 12;D, the protrusion is complete, × 6. In each casearepresents the Nematode, andbits protruded uterus.
Fig. 76.—Four stages in the life-history ofSphaerularia bombiDufour, ♀. (From Leuckart.)A, Beginning of the protrusion of the uterus (b), × 66;B, later stage, × 66;C, later stage, × 12;D, the protrusion is complete, × 6. In each casearepresents the Nematode, andbits protruded uterus.
Fig. 76.—Four stages in the life-history ofSphaerularia bombiDufour, ♀. (From Leuckart.)A, Beginning of the protrusion of the uterus (b), × 66;B, later stage, × 66;C, later stage, × 12;D, the protrusion is complete, × 6. In each casearepresents the Nematode, andbits protruded uterus.
Atractonema gibbosum, which lives in the body-cavity of the larva ofCecidomyia pini, has a similar life-history, but the parasitic form has a structural peculiarity which merits attention (Fig. 75). At the time of sexual maturity a swelling, which is caused by the prolapsus of the uterus and vagina, appears at the posterior end of the body; this swelling increases until it equals the rest of the body of the Nematode in size. Even this is far surpassed by a similar protuberance inSphaerularia bombi, where the evaginated sac grows with such extreme rapidity that in a few weeks its length increases from .25 mm. to 15 mm. and its volume 60,000-fold, the increase being due, according to Leuckart, to the increase in size of the individual cells and not to their multiplication. The Nematode which has produced this enormous growth gets relatively smaller and smaller, and ultimately drops off (Fig. 76). The sexual larvae which arise from the eggs in this sac leave the body of the bee in which this species is parasitic by the anus,and may live in damp earth, moss, etc., for months without taking nourishment, until the autumn, when they become sexually mature and, according to Leuckart, pair. The fertilised female is believed to bore her way into the humble-bee whilst the latter is seeking her underground winter quarters; this accounts for the fact that only queen bees are infected. The parasite is widely distributed both in Europe and North America; it is found in many species ofBombus, but most frequently inB. lapidariusandB. terrestris. The presence of theSphaerulariaaffects the reproductive organs of the host, and reduces their fertility, so that an infected queen bee never succeeds in forming a colony.
VI. Family Anguillulidae.
For the most part free living and of small size. The oesophagus has usually a double swelling or two oesophageal bulbs. The male has two equal spicula.
Genera:Diplogaster,Mononchus,Rhabditis,Tylenchus,Anguillula, and many others.
Many species of this family live in humus or decaying matter; others live on, or are parasitic in, plants; some, such asAnguillula aceti, which is found in vinegar and in paste, live in organic fluids.
The part played by the presence of these Nematodes in the soil is not thoroughly understood; sometimes they occur in great numbers, and even when not directly parasitic in plants, probably do them much damage. Cobb[190]has recently described from Australia and Fiji over eighty species, one-half of them new, which occur mostly in the earth, and many of them among plant roots. They frequently crawl up on to plants, especially on to seedlings. An instance of this is given as follows: "The edible part of three bunches of nice-looking celery bought of a Chinaman in Sydney was cut off as far up as it was tender, nearly to the first leaflets. It was washed by hand in a tin dish in tank water, free from Nematodes. The washings gave about 200 to 300 Nematodes, belonging to five different genera."
It is very probable that many of the free-living forms which have received distinct specific names may ultimately turn out to be but stages in the life-history of some of the parasitic species.Von Linstow[191]has pointed out that the free form ofA. diplogaster, if found alone, would be placed in the genusDiplogaster; similarly the bisexual form ofAscaris nigrovenosais known asRhabditis nigrovenosa.
Those Nematodes which live parasitically in plants,e.g.many of the generaTylenchusandAphelenchusandHeterodera, as well as those which only pierce the epidermis of the roots (the remaining species of the above-named genera), are provided with a spine which works to and fro through the mouth and assists the animal to bore into the tissues of the plant.Tylenchus devastatrixlives and reproduces in leaves and stems (never in the roots, except in the case of hops[192]) of many cultivated plants, such as rye, oats, onions, etc. "Clover sickness" is probably caused by this Nematode. The plants become infected by the thread-worms in the soil during the spring; their presence causes swellings and often kills the plant, in which case the worms return to the soil or remain in the straw.
Tylenchus triticiNeed. is the cause of "ear-cockles" in corn. These take the form of brown or purple galls, which replace the grains of corn, and which contain hundreds of minute Nematodes. In these galls they are motionless, and are capable of surviving in dryness for at least twenty years; but when moistened,—for instance, by the gall falling on damp earth,—they resume their vitality and make their way to the young wheat plants, and then, wriggling up the leaves and stems, find their way to the ear. Here they pair, and producing a gall-like growth in the flower, lay numerous eggs, from which arise the Nematodes of the ear-cockle.
fig77Fig. 77.—A,a, FemaleHeterodera schachtiiSchmidt, 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.)
Fig. 77.—A,a, FemaleHeterodera schachtiiSchmidt, 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.)
Fig. 77.—A,a, FemaleHeterodera schachtiiSchmidt, 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.)
Heterodera schachtii[193]Schmidt, is the cause of the "beetsickness," and forms galls or swellings on the roots of many plants, in England especially on the roots of tomatoes and cucumbers. The free larvae live in the earth and make their way into the smaller rootlets; here the female larvae shed their skin, lose their characteristic Nematode form, and become citron-shaped (Fig. 78, D). The male larvae undergo a change, and after a period of rest cast their skin and, leaving the rootlet, seek out the females. The female does not undergo this second ecdysis, but its generative organs grow and mature in what is practically a larval stage. The embryos develop within the body of the mother, and, escaping through the uterus, ultimately cause her death. They then make their way into the earth. The cycle of the development takes but four or five weeks, so that, as in the case ofTylenchus devastatrix, there are several broods in a year;T. tritici, on the other hand, has but one.
fig78Fig. 78.—A, MaleHeterodera schachtiistrongly magnified;a, head lappets;b, mouth cavity;c, spine;d, muscle of spine;e, gland;f, oesophagus;g, bulb;h, nerve-ring;i, excretory pore;j, intestine;k, testis;l, intestine;m, muscles moving spicule;n, 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;a, boring apparatus;b, oesophageal bulb;c, excretory pore;d, alimentary canal;e, anus;f, ovary:E, a male shortly before casting its larval skin.
Fig. 78.—A, MaleHeterodera schachtiistrongly magnified;a, head lappets;b, mouth cavity;c, spine;d, muscle of spine;e, gland;f, oesophagus;g, bulb;h, nerve-ring;i, excretory pore;j, intestine;k, testis;l, intestine;m, muscles moving spicule;n, 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;a, boring apparatus;b, oesophageal bulb;c, excretory pore;d, alimentary canal;e, anus;f, ovary:E, a male shortly before casting its larval skin.
Fig. 78.—A, MaleHeterodera schachtiistrongly magnified;a, head lappets;b, mouth cavity;c, spine;d, muscle of spine;e, gland;f, oesophagus;g, bulb;h, nerve-ring;i, excretory pore;j, intestine;k, testis;l, intestine;m, muscles moving spicule;n, 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;a, boring apparatus;b, oesophageal bulb;c, excretory pore;d, alimentary canal;e, anus;f, ovary:E, a male shortly before casting its larval skin.
Vuillemin and Legrain[194]point out that whileHeteroderais injurious to cultivated plants growing in damp soil, its presence is advantageous to those that grow in deserts. It is very common in the Sahara, and attacks many plants which areimmune from it elsewhere. It causes the rootlets to swell out, and the bladder-like extensions thus formed act as reservoirs for water.
Many other species attack plants;Tylenchus millefoliiLöw forms galls onAchillea,T. dipsaciKühn. on the teazle. They all seem to have great powers of resisting desiccation. The former species, when dried and placed in a herbarium in May, gave rise to active worms when moistened the following October; and the corn eel-worm is said to survive twenty-seven years in a state of suspended animation. On the other hand, although these Nematodes like moisture, they cannot withstand submersion in water for any time. They can resist a considerable degree of cold, and a species,Aphelenchus nivalisAuriv.,[195]has been described from Spitzbergen, where it lives in the snow amongst a small red alga,Sphaerella nivalis.
VII. Family Enoplidae.
Small, as a rule free-living, usually marine Nematodes, without a second oesophageal bulb. Eyes and mouth-armature often present. Fine hairs and bristles sometimes surround the mouth.
Genera:Enoplus,Dorylaimus,Enchelidium, and others.
The genusEnoplusis exclusively marine, living amongst Algae and Hydroids in shallow water and moving actively about, but never coiling into spirals. De Man[196]describesEnoplus brevisBast. as being attacked by a plant parasite, probably a Bacterium, of a greenish colour, which infested the muscles and gave them a peculiar colour.
Numerous other species have been described by De Man from the coast of Holland. It is probable that some of them are the free stages of parasitic forms; a brackish water species found in the East Indies (Dorylaimus palustris) is regarded by Carter as the larva ofFilaria medinensis.Oncholaimus echiniLeyd. is parasitic in the intestine of the sea-urchinEchinus esculentus.Tricoma cincta[197]has a strongly striated cuticle, which gives it almost the appearance of segmentation.Fimbria tenuishas numerous hairs on the tail, and the mouth is surrounded by bristle-bearing papillae.