Here must be mentioned two families closely allied to the true Nematodes.
(i.)Chaetosomatidae.—This family includes three genera:Chaetosoma,Rhabdogaster, andTristicochaeta. According to Metschnikoff,[198]although they are not true Nematodes, they have a great likeness to the group. He distinguishes them from the swimming members of the group as "creeping Nematoda."Chaetosoma, of which two species are known,C. ophicephalumandC. claparedii, has a head distinct from the body (Fig. 79). The mouth is at the anterior end, surrounded by a double semicircle of movable spicules; the whole body is covered by fine hairs, and on the ventral surface, just in front of the anus, is a double row of about fifteen cylindrical projections, by whose agency the animal creeps. The femaleC. claparediiis 1.5 mm. long, the male 1.14 mm. They were found creeping about on sea-weeds in the neighbourhood of Salerno.
fig79Fig. 79.—Mature female ofChaetosoma claparediiMetschni., × 57. (From Metschnikoff.)a, Oesophagus;b, intestine;c, anus;d, ovary;e, generative pore;f, ventral bristles.
Fig. 79.—Mature female ofChaetosoma claparediiMetschni., × 57. (From Metschnikoff.)a, Oesophagus;b, intestine;c, anus;d, ovary;e, generative pore;f, ventral bristles.
Fig. 79.—Mature female ofChaetosoma claparediiMetschni., × 57. (From Metschnikoff.)a, Oesophagus;b, intestine;c, anus;d, ovary;e, generative pore;f, ventral bristles.
The genusTristicochaeta[199]differs from the foregoing in having three rows of locomotor projections instead of two.
fig80Fig. 80.—Tristicochaeta inarimensePanceri, in one of its most usual positions, showing the triple row of ventral bristles, × 100. (From Panceri.)
Fig. 80.—Tristicochaeta inarimensePanceri, in one of its most usual positions, showing the triple row of ventral bristles, × 100. (From Panceri.)
Fig. 80.—Tristicochaeta inarimensePanceri, in one of its most usual positions, showing the triple row of ventral bristles, × 100. (From Panceri.)
Rhabdogasterhas no head distinct from the body, though the anterior part of the body is swollen. A second swelling occurs, as is also the case withChaetosoma, in the region of the opening of the genital ducts. The female inRh. cygnoidesattains a length of 0.36 mm. In this genus the hairs are confined tothe dorsal middle line. The locomotor projections are hooked, and are much finer than those ofChaetosoma, and they are situated farther forward than in the last-named genus.Rhabdogasteroccurs in the same surroundings asChaetosoma.Ch. ophicephalumis recorded from the English Channel.
(ii.)Desmoscolecidae.—The members of this family are minute, and are characterised by the presence of well-marked ridges which surround the body and give it an appearance of segmentation. The head, which is somewhat swollen, bears four bristles, and single pairs are borne by a certain number of the ridges, some on the dorsal and some on the ventral surface. These hairs can be moved independently of one another. Two red eye-spots are described between the fourth and fifth rings. The sexes are distinct, and the internal organs generally have a marked resemblance to those of the true Nematoda. TheDesmoscolecidaemove by looping their bodies after the manner of the Geometrid caterpillars, as well as by creeping with their bristles. The genus contains numerous species[200]:D. minutusClap. (English Channel),D. nematoidesGreef,D. adelphusGreef,D. chaetogasterGreef,D. elongatusPanceri, andD. lanuginosaPanceri. They are exclusively marine.
fig81Fig. 81.—FemaleDesmoscolex elongatusPanceri, ventral view, × 260.a, Ovary. (From Panceri.)
Fig. 81.—FemaleDesmoscolex elongatusPanceri, ventral view, × 260.a, Ovary. (From Panceri.)
Fig. 81.—FemaleDesmoscolex elongatusPanceri, ventral view, × 260.a, Ovary. (From Panceri.)
Trichoderma oxycaudatumGreef[201]is a minute animal, 0.3 mm. long, which has no head or ventral spines, but whose body is ringed and covered with long hair-like bristles. The male has two spicules, and the internal organisation recalls that of other Nematodes; still its ringed body has induced some authorities to place it near toDesmoscolex.
The Life-History of Nematodes.
Although, considering the enormous number of species of Nematodes and the remarkable diversity of the conditions underwhich they live, their bodily structure shows a very striking uniformity, the same is by no means the case with their life-history, which exhibits an astounding variety. Von Linstow[202]has arranged the various modifications, which occur under fourteen heads. He includes in his list the Gordian worms, which we have placed under a different heading. The following account has been taken from his paper, with a few alterations:—
1. The embryos develop, with a larval stage and without any change of medium, directly into the mature sexual forms. They live in fresh, brackish, or salt water, in plants, in the earth or in decaying organic matter: examples,Dorylaimus,Enoplus,Plectus,Monhystera.
2. The larvae live in the earth, the sexual forms in plants: examples,Tylenchus triticiandT. devastatrix,Heterodera schachtii(Figs. 77 and 78).
3. The larvae live in animals, after whose death and decay they are set free and develop into the sexual animals in the earth: example,Rhabditis pellio.
4. The bisexual forms live in the earth, and the fertilised females bore into animals (insects), and here produce embryos: example,Sphaerularia bombi(Fig. 76).
5. The bisexual forms live in the earth; the females do not develop, but the males make their way into Insects (Beetles), and becoming hermaphrodite, develop ova which give rise to the bisexual form: example,Bradynema rigidum.
6. The larvae live in the earth, the sexual form in Vertebrates: examples,Dochmius,Strongylus.
7. The Nematode lives as a hermaphrodite in animals, the offspring of this, by an alternation of generations, become sexual in the earth: example,Rhabdonemain Frog.
8. A bisexual free form gives origin to a bisexual parasitic form living in an animal: example,Leptodera appendiculatain Snails.
9. The eggs develop in the earth, and give rise to embryos which are transferred whilst still in the egg-cell to the body of an animal. The embryos hatch out and form bisexual parasites: examples,Oxyuris,Trichocephalus.
10. The larvae live in insects, the sexual worms in water or in the earth: example,Mermis.
11. The larva lives encapsuled and is passively transferred toa second animal: examples,Ollulanus, from Mouse to Cat;Cucullanus elegans, fromCyclopsto Perch;Spiroptera obtusa, from Meal-worm to Mouse.
12. The sexual form lives for a short time in the intestine of a Vertebrate, and produces larvae which bore through the intestinal wall and become encapsuled in the tissues: example,Trichina spiralis.
13. The sexual animal lives in the trachea of birds; the ova containing embryos are coughed up and are taken into other birds with food. They quit the egg-shell and wander into the air-sacs, and finally into the trachea: example,Syngamus.
14. There are two larval forms; the first lives in water, the second in the lungs of Amphibia, whence they wander into the intestine and become sexually mature: example,Nematoxys longicaudainTriton alpestris.
Parasitism.
1.Effect of Parasitism on the Parasite.—The usual effect of parasitism on the parasitic organism is that the various organs necessary for a free life tend to degenerate, whilst there is a multiplication and development of organs of adhesion, by means of which the parasite maintains its hold on its host. There is further an immense increase in the powers of reproduction, which may take the form of an increase in the number of fertilised eggs produced, or the parasite may at some time of its life reproduce asexually, by budding, or fission, or parthenogetically.
Of the various classes of animals which are more or less parasitic, the Nematodes show less difference between the free-living and parasitic members of the group than obtains in any other class. With few exceptions, such asSphaerularia,Allantonema, and one or two others, the parasitic forms have undergone but little degeneration. It is true that they have no eyes such as the free forms often possess, but in other respects, such as in the nervous, muscular, and digestive systems, they do not show any marked retrogression; further, the mouth-armature is developed in many free forms, and is not confined to the parasites.
The group has developed no methods of asexual reproduction by budding or fission, such as are found in Platyhelminthes; and the cases of an alternation of generations in which a sexual form alternates with a parthenogenetic form, are rare,e.g.Rhabdonema nigrovenosum; and it seems possible that even when parthenogenesis has been described, further observation may show that the parthenogenetic stage is really a protandrous hermaphrodite, in which case the alternation of generations in Nematodes,i.e.the hermaphrodite alternating with the dioecious form, is a case ofheterogamyor the alternation of two sexual generations.
On the other hand, parasitic Nematodes produce enormous numbers of eggs. Van Beneden states that 60,000,000 have been computed in a single Nematode, and this multiplication of ova is absolutely necessary, for the chance of the embryo reaching the right host, in which alone it can develop, is always a small one.
It is a common thing to find that parasites are either hermaphrodite or that the male is degenerate, as is the case with many of the parasitic Crustacea, but with one or two exceptions the Nematoda are bisexual, and although, as a rule, the males are smaller than the females, they show no other trace of degeneracy.
In spite of the fact that the class as a whole shows but few special modifications consequent on a parasitic mode of life, it is clear that the Nematoda are peculiarly adapted for such a mode of life. Their elongated thread-like bodies afford little resistance to the passage of the food, which, as it passes through the intestine of the host, might tend to carry the parasites out of the body. At the same time their shape enables them to pierce and wriggle through the various tissues without making any very serious lesions such as might prove fatal to their host. Their extraordinary power of resisting desiccation both in the egg and in the adult state vastly increases their chances of ultimately hitting on the right host. They are capable of living in a state of suspended animation for months, and even years when dried (videp.136), and of resuming their activity on being moistened.
The great faculty this group shows for living parasitically is evinced by the extraordinary variety of life-history presented by the different species. There is scarcely a stage which may not be parasitic; the eggs, the larvae, the adults are all in some cases free, in others parasitic, and in many cases first the one and then the other.
2.Occurrence and Effect of the Parasite on the Host.—Von Linstow states that the only law that can be derived inductively from the study of the life-history of Nematodes is that those that live in animals never pass through all their stages of development in the same organ; consequently, in considering the distribution ofthe parasites within the body of their host we have a double habitat to consider. Many forms, such asTrichina spiralis, wander from the intestine to the muscles; others, such asFilaria medinensis, from the alimentary canal to the lymphatics or blood vessels or subcutaneous tissues. Others pass from the body-cavity to the intestine, as the Mermithidae, which infest Insects, or from the stem and leaves of a plant to its flower, as in the case ofTylenchus tritici.
With regard to their occurrence in the different classes of the animal kingdom, they have been most frequently observed in Vertebrates and in Insects. They are comparatively rare in the other large divisions. Many genera are confined to certain hosts: thusAscaris,Filaria,Trichosomaoccur only in Vertebrates;Spiroptera(with one exception) in Mammals and Birds;Cucullanusin Fishes and Amphibia;StrongylusandPhysalopterain Mammals, Birds, and Reptiles;Dochmius,Pseudalius,Trichocephalusin Mammals;Dispharagus,Hystrichis,Syngamusin Birds;Nematoxys,Hedrurisin Amphibia and Reptiles;Ichthyonemain Fishes; andIsacisandMermisin Insects.
Twenty-two species have been described as parasitic in man, of which perhaps the most dangerous areFilaria medinensis, the three varieties ofF. sanguinis hominis;Dochmius(Ancylostomum)duodenalis, andTrichina spiralis. The Ascaridae, asAscaris lumbricoidesandOxyuris vermicularis, though painful, seldom cause death.
The enormous number of parasites harboured by one host is shown by the fact mentioned in Leuckart'sParasites of Man, that Nathusius[203]took from a single black stork 24 specimens ofFilaria labiatafrom the lungs, 16Syngamus trachealisfrom the trachea, more than 100Spiroptera alatafrom the coats of the stomach, besides several hundred Trematodes belonging to several different species (see p.63). Even this has been surpassed in the case of a young horse, in whose body Krause found 500Ascaris megalocephala, 190Oxyuris curvula, several millions ofStrongylus tetracanthus, 214Sclerostomum armatum, 287Filaria papillosa, 69Taenia perfoliata, and 6Cysticercusforms.
It is impossible here to enter into a full description of the destruction caused to domesticated animals and crops by the presence of these parasites; full details will be found in books dealingespecially with this question, such as Neumann'sParasites and Parasitic Diseases of Domesticated Animals. A couple of cases will show how important this matter is to the farmer. Crisp estimates thatSyngamus trachealiscauses the death of half a million pullets in England every year, and Mégnin states that in a single pheasantry 1200 victims died daily; again, the loss of one-third the crop of beetroot is by no means uncommon when it is infested withHeterodera schachtii. These show the practical importance of what at first sight seem quite insignificant animals, and the necessity for the minutest observation, for only when we are fully acquainted with all the details of the life-history of a parasite are we in a position to successfully combat it.
Sub-Order II. Nematomorpha.
Until the last few years it has been customary to regard the Gordiidae as a family of Nematodes. Although in external appearance and life-history they closely resemble the members of this group, yet recent research has shown so many important morphological differences between them and the Nematoda, that most zoologists are now agreed in placing them in a different sub-Order, the Nematomorpha, a name first suggested by Vejdovsky.[204]
fig82Fig. 82.—A water plant around which a femaleGordiusis twining and laying eggs.a,a, Clump and string of eggs. (From von Linstow.[205])
Fig. 82.—A water plant around which a femaleGordiusis twining and laying eggs.a,a, Clump and string of eggs. (From von Linstow.[205])
Fig. 82.—A water plant around which a femaleGordiusis twining and laying eggs.a,a, Clump and string of eggs. (From von Linstow.[205])
The Gordiidae comprise but two genera,GordiusandNectonema. The latter has but one species,N. agileVerr., and is marine; the former, on the other hand, is exclusively fresh-water, and contains a very large number of species. Gordian worms are frequently to be found in ditches, ponds, or large puddles, moving with an undulating motion through the water, or twining and writhing round water-plants; they are scarcer in running water. In shape they are like a piece of thin whip-cord, slightly taperingat each end; the male, however, is easily distinguished from the female by its forked tail (Fig. 89). Not unfrequently a considerable number are found inextricably tangled together into a knot, and the name of the genus refers to this fact. Where numbers have suddenly appeared in water hitherto free from them, legends have sprung up which attribute their presence to a rain of worms; in reality they have come out of the bodies of Insects in which they are parasitic for the greater part of their life.
The genusGordiuspasses through three distinct stages, of which the first two are larval and parasitic; the third is sexually mature and lives in water. The second larval stage closely resembles the adult, but the reproductive organs are not developed. The following account of the structure of this larval form and of the adult is in the main taken from von Linstow.[206]
The whole body is covered with a well-developed two-layered cuticle, which in the adult is marked out into areas, and bears numerous minute sensory bristles, which are especially developed in the neighbourhood of the cloaca of the male. Beneath this is a hypodermis which differs markedly from the sub-cuticle of Nematodes, inasmuch as it consists of a single layer of polygonal nucleated cells. Within this lies a single layer of longitudinal muscle-cells, which differ from the corresponding layer of Nematodes in having that part of their medulla which is not surrounded by the contractile portion directed outwards towards the hypodermis, and not inwards towards the body-cavity.
fig83Fig. 83.—Transverse section through a young maleGordius tolosanusDuj. (From von Linstow.) Highly magnified.a, Cuticle;b, hypodermis;c, muscular layer;d, parenchyma;e, alimentary canal;f, nervous system;g, cells of the testis.
Fig. 83.—Transverse section through a young maleGordius tolosanusDuj. (From von Linstow.) Highly magnified.a, Cuticle;b, hypodermis;c, muscular layer;d, parenchyma;e, alimentary canal;f, nervous system;g, cells of the testis.
Fig. 83.—Transverse section through a young maleGordius tolosanusDuj. (From von Linstow.) Highly magnified.a, Cuticle;b, hypodermis;c, muscular layer;d, parenchyma;e, alimentary canal;f, nervous system;g, cells of the testis.
The body is in the younger stages practically solid, the interior being filled with clearly defined polygonal cells which are arranged in definite rows; in later life certain splits arise in this tissue whichsubserve various functions; between these splits strands of tissue are left which form mesenteries, and some of the cells remain lining the muscular layer (Fig. 86). These cells have been described by Vejdovsky as a definite somatic, peritoneal epithelium, but this was not found by von Linstow. Besides forming the mesenteries, and acting as packing between the various organs of the body, these cells also form the ova and the spermatozoa.
The splits which have appeared when the animal has reached the second larval stage, are two dorsal and a ventral; the latter contains the alimentary canal, and may be termed the body-cavity, the former will develop the generative organs. The mouth is occluded in the older larvae, and in the adults there is a distinct but solid oesophagus which passes into a tubular intestine. The intestine consists of a single layer of cells surrounding a lumen; it runs straight to the hinder end of the body, where it opens in both sexes with the ducts of the reproductive organs.
The nervous system consists of a well-defined circumoesophageal ring with two dorsal swellings, and, arising from this, a median ventral cord which runs the whole length of the body. The cord consists of three longitudinal strands with ganglionic cells below them; the latter, though they lie within the muscle layer, maintain a connexion with the hypodermis. Behind, the nerve-cord splits in the male, one half passing into each caudal fork. In the adult a pair of black eyes can be detected on the head; the only other sense organs are the tactile bristles mentioned above. Excretory organs are unknown.
fig84Fig. 84.—Section through a young femaleGordius tolosanus. (From von Linstow.)a, Cuticle;b, hypodermis;c, muscular layer;d, parenchyma;e, alimentary canal;f, nervous system;g, egg-sac;h, ovary.
Fig. 84.—Section through a young femaleGordius tolosanus. (From von Linstow.)a, Cuticle;b, hypodermis;c, muscular layer;d, parenchyma;e, alimentary canal;f, nervous system;g, egg-sac;h, ovary.
Fig. 84.—Section through a young femaleGordius tolosanus. (From von Linstow.)a, Cuticle;b, hypodermis;c, muscular layer;d, parenchyma;e, alimentary canal;f, nervous system;g, egg-sac;h, ovary.
The generative organs only attain maturity in the adult, which is, in fact, exclusively devoted to reproduction. No trace of testesis found in the larva, though the two dorsal splits from the walls of which the spermatozoa will arise are present. They are lined by a definite epithelium (Fig. 83), and this serves at once to distinguish them from the body-cavity. Posteriorly the splits narrow and become the two vasa deferentia which open one on each side into the cloaca. The cells lining the lumen give rise to secondary cells, and these become spermatozoa, the process extending from behind forwards. The external organs—bursa, etc.—described by Vejdovsky were not found by von Linstow.
fig85Fig. 85.—Section through a mature femaleGordius tolosanus. (From von Linstow.) Lettering as in Fig. 84;g, egg-sac;h, ovary.
Fig. 85.—Section through a mature femaleGordius tolosanus. (From von Linstow.) Lettering as in Fig. 84;g, egg-sac;h, ovary.
Fig. 85.—Section through a mature femaleGordius tolosanus. (From von Linstow.) Lettering as in Fig. 84;g, egg-sac;h, ovary.
fig86Fig. 86.—Section through a femaleGordius tolosanuswhen the deposition of ova is almost complete.a,b,c,d,e, andf, as in Fig. 84;g, egg-sac;h, ovary almost empty;i, dorsal canal containing eggs;j, receptaculum seminis.
Fig. 86.—Section through a femaleGordius tolosanuswhen the deposition of ova is almost complete.a,b,c,d,e, andf, as in Fig. 84;g, egg-sac;h, ovary almost empty;i, dorsal canal containing eggs;j, receptaculum seminis.
Fig. 86.—Section through a femaleGordius tolosanuswhen the deposition of ova is almost complete.a,b,c,d,e, andf, as in Fig. 84;g, egg-sac;h, ovary almost empty;i, dorsal canal containing eggs;j, receptaculum seminis.
In the female larva two similar splits are present; these form the egg-sacs. Posteriorly they end in two short oviducts which open into a uterus, in which fertilisation takes place, and in which the secretion arises which cements the eggs together. In the adult the ovaries and a receptaculum seminis are found, in addition to the organs present in the larva. The ovaries are formed from modifications of the packing tissue; they begin close behind the head, and soon attain such dimensions as to compress the egg-sacs and body-cavity to small slits. After a time the wall between the ovary and the egg-sacs becomes absorbed, and the eggs grow into the latter. In the old females, where the egg sacs are empty, there is a considerable space round the exhausted ovary, into which eggs continue to fall off; thereis also a median dorsal canal which contains a few eggs. By this time the wall between the ovary and the egg-sac has again appeared.
One of the most interesting points about the female is that, according to Vejdovsky, the ovary is segmented, the cells which form the ova being heaped up in segmentally-arranged masses. This observation, if correct, is almost the only instance of segmentation recorded in the group Nemathelminthes.
fig87Fig. 87.—Nectonema agileVerrill.A, The adult. Magnified. (After Fewkes.)B, Longitudinal section through the head. × about 20. (From Bürger.)a, Mouth;b, circumoesophageal commissure (dorsal);c, cell of salivary gland;d, septum cutting off head from rest of body;e, testis;f, ventral cord;g, oesophageal cells;h, lumen of oesophagus;i, cerebral ganglion (ventral).
Fig. 87.—Nectonema agileVerrill.A, The adult. Magnified. (After Fewkes.)B, Longitudinal section through the head. × about 20. (From Bürger.)a, Mouth;b, circumoesophageal commissure (dorsal);c, cell of salivary gland;d, septum cutting off head from rest of body;e, testis;f, ventral cord;g, oesophageal cells;h, lumen of oesophagus;i, cerebral ganglion (ventral).
Fig. 87.—Nectonema agileVerrill.A, The adult. Magnified. (After Fewkes.)B, Longitudinal section through the head. × about 20. (From Bürger.)a, Mouth;b, circumoesophageal commissure (dorsal);c, cell of salivary gland;d, septum cutting off head from rest of body;e, testis;f, ventral cord;g, oesophageal cells;h, lumen of oesophagus;i, cerebral ganglion (ventral).
The only other genus which is associated withGordiusin the group Nematomorpha isNectonema, of which there is as yet but one species known,Nectonema agileVerr.[207]Our knowledge of the anatomy of this worm is due mainly to Bürger[208]and Ward.[209]Nectonemais a marine worm found swimming near the surface of the sea with rapid undulatory motion. The males are from 50 to 200 mm. long, the females from 30 to 60 mm. The body is faintly ringed, and bears two rows of fine bristles on each side. Owing to a curious torsion of the body through a right angle, the lateral bristles of the anterior third seem to be placed in the ventral and dorsal middle line. They are very easily broken off. The body is divided into a small anterior and a large posteriorchamber by a transverse septum placed a little way behind the head. The anterior chamber contains the brain and is lined by a definite epithelium, the posterior is not. The layers of the skin correspond with those of Nematodes or ofGordius, but the hypodermal cells show no cell outlines; still they are not so modified as in the former group. The hypodermis is thickened in the median dorsal and ventral line, and the single nerve-cord lies in the latter.
The alimentary canal is degenerate, as inGordius. A mouth exists, but it is minute, and opens into a very fine tube lined with chitin, which pierces through the substance of a single elongated cell. This minute oesophagus, with its coextensive cell, reaches back to the transverse partition, but behind this a few other cells become associated with it, and ultimately the lumen of the alimentary canal is surrounded by four cells; but the number diminishes behind, and soon only two cells surround the tube at any one level, and the intestine dwindles away some little distance in front of the tail. There is no sign of an anus. A circumoesophageal nerve-ring exists, of which the ventral part is by far the larger (Fig. 87); it gives off a ventral nerve-cord, which swells posteriorly in the male into a large anal ganglion, far bigger than the brain, and larger in the male than in the female.
The testes consist of a dorsally placed sac, continuous behind with a vas deferens; this opens at the posterior end, which is pointed and slightly curved ventrally. The ovary is unknown; but females have been found with their body-cavity crammed with ova; these escape, like the spermatozoa, from a genital pore at the posterior end of the body.
Classification.—The separation of the Nematomorpha from the Nematoda depends mainly on the character of the nervous system, the absence of the lateral lines and of the dorsal line, the character of the contents of the body-cavity, and the character of the reproductive organs. In Gordiidae the latter are always placed dorsal to the intestine, and ovaries and testes open alike at the hinder end of the body. The importance of the differences in the organs just enumerated has been considered sufficient to justify the removal of the Gordiidae from the Nematoda, and the establishment of the special sub-Order Nematomorpha for their reception; and althoughNectonemahas a dorsal line, and is in some other respects intermediate between the two groups, there can be little doubt that it is more closely allied toGordiusthan to any memberof the Nematoda, and it must therefore be placed with it in the Nematomorpha.
On the other hand, it ought to be mentioned that Camerano[210]found that the chief details of the fertilisation and development of the egg inGordiusclosely conform with what is known of the same processes in Nematodes, and he is of opinion that these resemblances are sufficiently important to justify the retention of the group among the Nematoda.
Life-History.—The life-history ofGordiuscomprises four stages—the early development of the egg, the first larval form, the second larval form, and the sexually mature form. Both larval forms are parasitic, and during their life they are actively engaged in feeding; the free form, on the other hand, takes in no nourishment, and is exclusively engaged in reproduction.
fig88Fig. 88.—Abdomen ofPterostichus nigerwith the terga removed to expose theGordiuslarva within. Slightly magnified. (From von Linstow.)
Fig. 88.—Abdomen ofPterostichus nigerwith the terga removed to expose theGordiuslarva within. Slightly magnified. (From von Linstow.)
Fig. 88.—Abdomen ofPterostichus nigerwith the terga removed to expose theGordiuslarva within. Slightly magnified. (From von Linstow.)
Von Linstow[211]gives the following account of the life-history ofG. tolosanus, a form which has been more fully worked out than any other. In the month of April numerous specimens of the beetlePterostichus nigerwere found floating on the surface of the ditches and small ponds in the fields surrounding Göttingen. Some were found dead or dying; others appeared quite healthy, and these were swimming actively, endeavouring to reach land. Within the abdomen of these beetles, in about 20 per cent of those collected, the second larval form of theG. tolosanuswas found. The longest larvae were 122 mm. in length, and very soft, partly snow-white and partly brown in colour; traces of the boring apparatus of the first larval form were still to be seen, but in other respects the larva only differed from the free form in the immaturity of its sexual organs. Besides the parasite hardly anything was to be found in the abdomen of the beetle, the larva having eaten up all trace of the fat body and the generative organs of its host. The larvae bored their way out of the body of the beetle and became adult animals.
It is rather difficult to say what brings these essentiallyterrestrial beetles to the water, but von Linstow suggests that, as they live partly on snails, and at this time of year there are not many land-snails about, they may be in search of water-snails such asLimnaea. They may also be sometimes blown into the water by wind storms, but, whatever the cause is, their presence in water is essential for the continuance of the life of their parasites.
Once free in the water theGordiusis soon sexually mature; the fertilisation takes place in April, and then the female may be seen twisting and writhing round the stems of water-plants and laying the long bead-like strands of eggs (Fig. 82). The first deposition observed by von Linstow took place on 14th April, the last on 2nd August, and the period of egg-laying for each female extended over four weeks. At first the eggs are snow-white, but within twenty-four hours they turn brown in colour.
The development of the first larva within the egg takes about a month. When it emerges from the egg-shell it is minute, .065 mm. long, ringed anteriorly, and provided with a protrusible and retractile boring apparatus consisting of three chitinous rods; round the base of this piercing proboscis is a double crown of papillae, each bearing a spine (Fig. 90).
fig89Fig. 89.—The tail ends of a femaleGordius(a) and a male (b)in copula. × 1.5. (From G. Meissner.[212])
Fig. 89.—The tail ends of a femaleGordius(a) and a male (b)in copula. × 1.5. (From G. Meissner.[212])
Fig. 89.—The tail ends of a femaleGordius(a) and a male (b)in copula. × 1.5. (From G. Meissner.[212])
This first larval form breaks through the egg-shell and sinks to the bottom of the water, where it moves about sluggishly and awaits the arrival of the right host in which to take up its abode. This host is the larva of the Alder-fly,Sialis lutariaLin. (videvol. v. p. 444), and into this it bores and comes to rest in the muscles or the fat body. It does not form distinct capsules. It remains in this larva during the following winter, and in the spring passes over into the imagoSialis. The complete insect frequents the small plants growing along the water's edge, and falls an easy prey to the predaceous beetlePt. niger. The larva is eaten, and undergoing a change becomes the second larval form mentioned above. It remains in the body of the beetle during the second winter, and finally returns to the wateras the adult some eighteen or twenty months after it has been hatched from the egg.
fig90Fig. 90.—Embryo or first larval form ofGordius tolosanustaken from the egg. Highly magnified.aandb, The bristle-bearing papillae on the head;c, the boring apparatus. (From von Linstow.)
Fig. 90.—Embryo or first larval form ofGordius tolosanustaken from the egg. Highly magnified.aandb, The bristle-bearing papillae on the head;c, the boring apparatus. (From von Linstow.)
Fig. 90.—Embryo or first larval form ofGordius tolosanustaken from the egg. Highly magnified.aandb, The bristle-bearing papillae on the head;c, the boring apparatus. (From von Linstow.)
From the above account of the life-history ofGordiusit will be seen that the chances of an egg reaching maturity are comparatively small, and to compensate for this a very large number of eggs are laid. In addition to the risk of the larvae not finding the right host at the right time, and of the first host not being eaten by the second, and the second not being drowned, there is the danger that the ditches and ponds in which the adults live may dry up, and, in fact, great numbers of worms perish by this taking place.
The sex of the adults may be told from their colour, the males being of a blackish brown, the females of a light clay brown; the former average 120 mm. in length, the latter 170 mm. The males are also more numerous, the proportion being seven to three. Camerano[213]has drawn attention to the fact that there is a certain polymorphism in size, form, and colour which is especially common amongst the males; dwarf forms with mature reproductive organs exist, and he is of opinion that these differences depend both on the size of the second host and on the duration of the parasitic life.
In addition to the larva ofSialis lutaria, the first larval stage has also been found in the larva ofEphemera,Tanypus,Corethra, andChironomus; the second inCarabus hortensisFabr.,Procerus(Carabus)coriaceusLinn.,Calathus fuscipesGoeze,Molops elatusFabr., several species ofPterostichus, and a number of other beetles. It is probable that its normal hosts areS. lutariaandPt. niger, but it is clear that it often comes to rest in other insects. The view that the Gordiidae have no special hosts, but may either pass the whole of their life-history within one and the same animal, or, on the other hand, may inhabit animals belonging to very different groups, is held by Villot, who has paid great attention to the subject. He finds the first larval form encysted in the walls of the alimentary canal in fishes, suchasLeuciscus phoxinus, the minnow,Cobitis barbatula, the loach, andPetromyzon planeri, the lamprey; in the larvae of Diptera,Ephemera, and beetles, inPlanorbis(a water snail), inEnchytraeus(an Oligochaet); the second larval form in all kinds of insects, spiders, Crustacea, fish, frogs, birds (Otis), and in man, and these various habitats lead him to the conclusion that "Les Gordiens n'ont pas d'hôtes spéciaux." On the other hand, as von Linstow points out, it is contrary to our knowledge of parasites that a single species should develop equally well in the body of warm and cold-blooded Vertebrates and of Insects, and the explanation of the presence of the larvae in these various forms may either be that they belong to different species ofGordiusor, more probably, that they are accidentally present, having passed into their hosts with drinking water.
fig91Fig. 91.—Tarsal joint of an Ephemerid larva into which twoGordiuslarvae (a,a) have penetrated. Magnified. (From G. Meissner.)
Fig. 91.—Tarsal joint of an Ephemerid larva into which twoGordiuslarvae (a,a) have penetrated. Magnified. (From G. Meissner.)
Fig. 91.—Tarsal joint of an Ephemerid larva into which twoGordiuslarvae (a,a) have penetrated. Magnified. (From G. Meissner.)
The number of species ofGordiusis large; over 100 are enumerated in theCompendium der Helminthologie,[214]the great majority of which inhabit insects.
The life-history ofNectonemais practically unknown; the adults have been found swimming near the surface of the sea at two places only: Newport, R.I., and Wood's Holl, Mass., on the south coast of New England. It has been fished close to the shore, from the end of June to the beginning of October, when the tide is going out at evening and there is no moon. This seems to indicate that it avoids the light. When first caught the worms move actively about, coiling themselves into figures of eight and then uncoiling; at the same time there is a rhythmical movement caused by waves of muscular contraction passing down each side of the body alternately; by this kind of motion they make rapid and definite progress through the water.
It seems probable that the adultNectonemais preceded by one or more larval stages, and what appears to be a young form hasbeen obtained from the thoracic cavity of a prawn,Palaemonetes,[215]which has thus some claim to be regarded as the host of this species, but nothing is known about its early life-history.
Sub-Order III. Acanthocephala.
The Acanthocephala, which form the third class of the Nemathelminthes, consists of but few genera; there are, however, numerous species of very different size, varying from 10 to 65 cm. long in the femaleGigantorhynchus(Echinorhynchus)gigas, to quite minute forms a few millimetres in length. The adult stage occurs in the alimentary canal of Vertebrates, as a rule in those which live in, or frequent water; the larvae are found in the bodies of certain Invertebrates, very frequently small Crustacea.
fig92Fig. 92.—Two specimens ofEchinorhynchus proteusWestrumb., with their anterior ends embedded in the wall of the intestine of a Pike. Magnified with a lens. (From Hamann.)
Fig. 92.—Two specimens ofEchinorhynchus proteusWestrumb., with their anterior ends embedded in the wall of the intestine of a Pike. Magnified with a lens. (From Hamann.)
Fig. 92.—Two specimens ofEchinorhynchus proteusWestrumb., with their anterior ends embedded in the wall of the intestine of a Pike. Magnified with a lens. (From Hamann.)
Anatomy.—The body of the mature forms can usually be divided into three sections—the proboscis, the neck, and the trunk, but the middle region is not always discernible. The proboscis is armed with rings of hooks (Fig. 93) arranged in longitudinal rows; they are usually of two kinds, but inE. proteusof three. They have a certain specific value, but not much stress can be laid on the number of rings,e.g.inE. angustatusthe number varies from eight to twenty-four. The recurved hooks serve to fasten the parasite very firmly to the tissues of the host. The proboscis is hollow and retractile; it can be withdrawn into the body by means of muscles attached internally to its tip. It does not, however, pass straight into the body-cavity, but is retracted into a special cavity—the proboscis sheath—with a double muscular wall. The proboscis sheath may perhaps be looked upon as a septum, such as is found in some of the Nematomorpha, dividing the body-cavity into two parts. It is inserted into the body-wall at the junction of the neck and trunk or of the proboscis and trunk. In addition to the muscles which withdraw the proboscis into its sheath, there are two retractors running from theoutside of the sheath to the body-wall; these serve to retract the whole sheath and its contents into the body-cavity of the trunk.
The structure of the skin is essentially like that of Nematodes, but the details are much more complicated. The whole body is covered by a thin cuticle secreted by the epidermis, which, as in the other groups, breaks down and forms a syncytium called the sub-cuticle. The minute fibrils which penetrate this layer are much more definitely arranged than in Nematodes; the largest of them run from without inwards, others run concentrically round the body. Large oval or spherical nuclei are scattered in the sub-cuticle, which is further honeycombed by a number of lacunae or spaces which are described below.
fig93Fig. 93.—A, Five specimens ofEchinorhynchus acusRud. attached to a piece of intestinal wall, × 4;B, the proboscis of one still more highly magnified.
Fig. 93.—A, Five specimens ofEchinorhynchus acusRud. attached to a piece of intestinal wall, × 4;B, the proboscis of one still more highly magnified.
Fig. 93.—A, Five specimens ofEchinorhynchus acusRud. attached to a piece of intestinal wall, × 4;B, the proboscis of one still more highly magnified.
Within the sub-cuticular layer is found a sheath of circularly-arranged muscle-fibres, and within this again a sheath of longitudinal muscles which do not extend into the proboscis; this inner layer lines the body-cavity, there being no epithelium within it. In their minute structure the muscle-cells resemble those of Nematodes.
The canals in the sub-cuticle form a very curious system of anastomosing spaces, in which a clear fluid containing fat globules circulates. The extent to which the system is developed varies in different species, but in all there is a pair of longitudinal canals which are situated laterally, and which give off the subsidiary channels in their course. The above description applies to the lacunar spaces in the skin of the trunk; those of the proboscis are quite distinct, and there is no communication between the two sets of spaces; in fact, the sub-cuticle in which the lacunae are formed is not continuous across the line of junction of the proboscis and the neck, or, when the latter is absent, of the proboscis and the trunk, but it is interrupted by the ingrowth of a thin ring of cuticle which reaches down to the muscular layers (Fig. 94).