There are other worms which migrate, and even some articulate animals; but their modifications of form are much fewer than in the preceding, and their changes are generally restricted to simple metamorphoses. We will place at the head of this chapter the Linguatulæ, which have so perplexed naturalists.
We sometimes find in the nasal fossæ of the dog and the horse a worm resembling a leech, with a body completely etiolated, which lives there entirely as a parasite, and whose history has only been known for a few years. Chabert discovered the first species of this group in 1787 in the frontal sinus of the horse and the dog. It had been named Tænia lanceolata. All naturalists, Cuvier included, placed this animal among intestinal worms, under the name of Linguatula or Pentastoma. The latter name had been given to it, because they mistook the hooks for mouths.
We have shown, from the embryos, in 1848, that the Linguatulæ, instead of being worms, are articulate animals, more allied to the lerneans or acaridæ than to the helmintha. These observations, though received at first with much hesitation, were fully confirmed afterwards, especially by the learned researches of Leuckart. The linguatulæ have a very long body, sometimes rounded, in other cases compressed, with a mouth surrounded by four strong hooks, regularly disposed in a semicircle. They have often been found in the lungs of serpents, in certain birds, and in many mammals. A linguatula was also seen by Bilharz at Cairo, in the liver of a negro, and they have been observed in the hospitals of Dresden and Vienna.
It is to be presumed that this dreadful parasite has been introduced into man by means of the flesh of the goat, and perhaps of the rabbit. Linguatulæ are found in their primary agamous form, in open cavities like the nasal fossæ. Leuckart was the first to show that the linguatulæ, which lived at first encysted in the peritoneum of the rabbit, completed their evolution and became perfect in the nasal fossæ of the dog. The Linguatula serrata (Fig. 65), which lives primarily in the goat, the guinea-pig, the hare, the rabbit, &c., is found accidentally in man, and perfect in certain mammals. Examples have been given of sick persons being completely cured by the evacuation of worms from the nostrils; these worms were, doubtless, linguatulæ. Fulvius Angelianus and Vincentius Alsarius speak of a young man who had suffered for a long time from head-ache, and who passed a worm from his nostrils. It was as long as the middle finger. There is little doubt that this was the Linguatula tænioïdes. These parasites may perhaps sometimes lose their way in their peregrinations. Some years ago a lioness died of peritonitis at Schönbrunn, and, after death, the liver, the spleen, and other organs were found to be filled with encysted linguatulæ.
Fig. 65.—Linguatula magnified six times. Four hooks are seen around the mouth in front. c, the anus.
The nematode worms are long and rounded, like the ordinary ascarides of infants, which take up their abode in all the organs of animals of the various classes of the animal kingdom. About a thousand varieties are known, varying in length from a few millimètres to forty or fifty centimètres.
They are not all parasites, as has been thought, since some are found in the sea, and others in damp earth, in putrid matter, and even on plants and their seeds. The migrations of nematodes are subjects of great interest. Their changes of form are usually not very considerable; but the modifications in their sexual apparatus, whether in the same individual, or in the succeeding generations, are very curious.
When we consider the numerous encysted and agamous nematodes, which are found in the different orders of mammalia, birds, reptiles, batrachians, and fishes, there is little doubt that all these beings are only migratory parasites, which pass together with their hosts into the animal to which they are destined. They are found, like ascarides, in animals of all classes. Some are to be met with in all the organs—the brain, the eye, the muscles, the heart, the lungs, the tracheal artery, the frontal sinus, the digestive tube, the skin, and even in the blood. Sometimes the two sexes live under the same conditions; sometimes the male is dependent on its female, or else one generation is parasitical, and the next is independent. There is a great diversity with respect to development. Some nematodes, like trichinæ, are developed so rapidly, that the embryos are already perfect in the egg before it has quitted its mother. Others, like the ascarides lumbricoides, lay eggs, in which the embryos do not appear till several weeks or many months after they have been laid. Between these two extremes we find all the intermediate degrees.
Diezing, who has done more for systematic helminthology than any other naturalist, brought together, under the name of Agamonema, all the migratory agamous nematodes which wait for the opportunity of entering their final host. Diezing had kept himself quite independent of the discussion by fixing his attention exclusively on form, without taking account of migration and digenesis. One of these agamonemata, lodged in the midst of a pediculated cyst on the vagina of a bat (the little horse-shoe), was probably a worm that has lost its way; if not, we must admit that these mammals become the prey of some carnivorous animal. But what carnivore can habitually feed on the cheiroptera? There are but few fishes, either in fresh or salt water, which do not enclose in the folds of their peritoneum, especially round the liver, cysts full of these agamonemata.
We see in some of the nematodes examples of migration which are quite peculiar to them. Some of these worms are always free, others free at one part of their life only, others migrate from one animal to another; others again from one organ to another. The Ascaris nigro-venosa of the frog lives sometimes in the lungs, at others in the rectum or quite out of the body in damp earth. The Filaria attenuata lives in the rook (Corvus frugilegus), and it is said that it becomes sexual in the intestines of the same bird.
These worms are usually very tenacious of life; many of them can, it is said, be dried for weeks, months, or years together, and return to life as soon as their organs are moistened. Their eggs resist even the action of alcohol and the most active chemical agents, and eggs that had been prepared for the microscope, and had served for many years the purposes of study, have been known to produce young ones as if they had been just laid.
Natura non facit saltus is especially true as to the division of sexes among the nematodes. Between the true hermaphrodites and the true diœcious worms are found species in which the males gradually dwindle and become dependent on the female; this is to be seen in the Sphœrulariæ, among which the male is only an appendage to the female sex. We find here full evidence of the fact that the female is more important than the male, with regard to the preservation of the species. In some species the sexes differ but little, in others, the sexual differences become greater, and the male is only one third of the length of the female; but in some of them the disproportion is greater still. At the same time, we see nematodes whose males are attached to the females, so as only to form a single individual; in other cases, the male seems to disappear to such an extent, that we find nothing but the male organ in the female; indeed, there are instances of male worms, which, without changing their form, occupy the cavity of the matrix and, like the lernean crustaceans, are parasites of their females. The Trichosomum crassicauda is an instance of this kind.
Arrangements which would not have been suspected beforehand, are every day revealed, with respect to the conservation of species. We have recently learned from the works of Messrs. Malmgren and Ehlers, and later still, from those of Claparède, that in the same species we may find different males, producing different offspring. Messrs. Malmgren and Ehlers have opened this question by their persevering researches, and Mons. Claparède expected to invalidate the results obtained by them by establishing himself at Naples, in order to devote himself to a new series of investigations. Contrary to his expectations, he arrived at the same conclusions, and announced that a nereid possesses, in one and the same species, two kinds of males and two sorts of females, and that these males differ from each other, not only in their manner of life but in their age, in the mode of formation of the spermatozoïds as well as in the form; that the females differ no less from each other than the males, and that each form is intended to provide, in its own manner, for the dissemination of the eggs.
We see this realized in annelid worms known by the name of Heteronereidæ. Certain individuals of small size live on the surface of the water; others, evidently much larger, live at the bottom of the sea and behave quite differently. The eggs and the spermatozoïds proceeding from these two forms differ sensibly from one another, and the difference of form corresponds with that of origin.
We see thus among some of them different males; among others different females: then eggs and spermatozoïds equally different in one and the same animal species.
A curious insect, the Termes lucifuga, appears also to distinguish itself by two sorts of males and females, which even take to flight at different periods. Great sagacity was required to reveal these strange facts. Mons. Lespes has had the courage to devote himself to these observations.
We see that all means are good that are for the preservation of the species, but who would have suspected that in a single animal there would be found two males by the side of two females, neither of which resembles the other, and besides these, two kind of eggs and spermatozoïds! How great would be our astonishment were we to see two sorts of cocks, two kinds of hens, and two sorts of eggs produced by the same mother, and hatched at the same time!
Professor Ercolani bred in damp earth certain parasitical nematodes, kept them alive, saw them reproduce, and was even able to obtain several generations of them. These nematodes were the Strongylus filaria from the lungs of the goat, the Strongylus armatus from the intestines of the horse, the Ascaris inflexa, and the Ascaris vesicularis from the fowl, and the Oxyuris incurvata from the horse. The first three, whether they are born in damp earth, or in the midst of organs in which they habitually lodge, have the same external characters; nothing is remarked in them except a greater activity in their reproduction.
The Strongylus armatus, when born at liberty, appears no longer to have hooks at the mouth like those worms which live in the intestines. Mons. Ercolani has also remarked that these worms, when they become free, are ovo-viviparous, though they were before oviparous.
There are many of these nematodes which are true parasites of man, and although certain of these are as much dreaded as the plague or the cholera, we are far from knowing all their history, and especially the manner in which they are introduced.
A young naturalist, Dr. O. Bütschli, has lately made a good résumé of the state of our present knowledge of parasitical and wandering nematodes.
The Sclerostomata are distinguished by their mouth being surrounded by a horny armature. The river perch usually gives lodging to a viviparous nematode, the Cucullanus elegans, on the development of which a special work has been published. The young ones are provided with a perforating stylet, and penetrate into the bodies of small aquatic crustaceans, called cyclops. When they have obtained entrance into this living lodging, they bore through the walls of the intestines and shut themselves up in the perigastric cavity. The cyclops being pursued by the young perch, are swallowed with their guest, and the latter is set free in the midst of the stomach, where it passes through its sexual evolution.
Leuckart saw in his aquarium young Cucullani penetrate into the bodies of the cyclops. These crustaceans are therefore the vehicle of these nematodes. Another nematode worm, the Dochmius trigonocephalus, lives at liberty while young, but seeks for an asylum in the dog in its old age. The Sclerostomum equinum causes aneurisms in the horse, which manifest themselves by colic. A hundred of these worms have been found in the same horse. The Sclerostomum pinguicola is very common in the pig in the United States. This is the Stephanurus dentatus of Diezing, noticed by Natterer in Chinese pigs in Brazil. Cobbold notices the same worm as living in the pig in Australia; they have been also found in Germany.
The Strongyli are round, cylindrical worms, with bodies sometimes entirely red, which inhabit different organs in mammals and birds. A very remarkable species, the Strongylus gigas (Fig. 66), exists in the kidneys of the horse and the dog, and sometimes in man. It partly destroys this organ, and has been seen a mètre in length. The Strongylus commutatus often lives in great abundance in the lungs of the hare, and the Strongylus filaria in the lungs of the sheep, occasionally in such great numbers that their presence produces pneumonia.
Fig. 66.—Strongylus gigas.—1, female, showing a, the mouth; b. the intestine; c, genital pore; d, anus. 2, cephalic extremity of the male; a, mouth; b, œsophagus. 3, caudal extremity of the male; a, cup; b, penis. 4, egg.
Porpoises generally have strongyli in their lungs and their bronchia, and they are seen by thousands in the sinus of the Eustachian tube. We collected a large bottle full from a single porpoise around its internal ear. When we consider the prodigious number of these creatures, may we not suppose that they are able to multiply in the organs which they occupy, as well as migrate to infest other individuals.
Different generic and specific names have been given to these Strongyli. A round worm found in the intestines of the dog, the Strongylus trigonocephalus, lives at first in damp earth or mud like the rhabdites in general; it then passes into the dog, and there becomes a sexual Strongylus. It is possible that there are others in the same category.
Fig. 67.—Ascaris lumbricoides.—1, complete worm, 2, head, 3, tail of the male, 4, middle of the body of female.
The Ascaris lumbricoides is a large round worm which attains the size of a quill pen, and which is commonly found in the stomach or the lesser intestines of children when in good health. Aristotle was acquainted with it. It has been observed throughout Europe, in Central Africa, in Brazil, and Australia. The same species lives in the intestines of the pig; but the Ascaris megalocephalus, which is usually found in the horse, is of a different species.
The Ascaris acus of the pike lives at first in a common white fish, the Leuciscus alburnus, and passes with this fish, which serves it as a vehicle, into its final host.
Another common nematode, the Oxyurus vermicularis (Fig. 69), a parasite of man, is a small worm of the size of a fine pin, which often multiplies in the rectum of children, causing intolerable itching. It is by means of their microscopic eggs that they penetrate into the system; these are hatched in the stomach, and are completely developed at the end of eight or ten days. They pass from the anus in great numbers.
Fig. 68.—Trichocephalus of man.—1, female, a, cephalic extremity, b, caudal extremity and anus, c, d, digestive tube and ovary, e, orifice of sexual apparatus. 2, isolated egg. 3, male, a, cephalic extremity, b, anus, c, digestive tube, d, spicula or penis, e, sheath into which it is withdrawn.
Fig. 69.—Oxyurus vermicularis.—1, male of natural size, 2, female, id., 3, cephalic extremity, magnified.
The brood of worms from the eggs of the Ascaris megalocephala of the horse live in freedom, and go through all their phases until their sexual development separately; there are males and females. The generation which descends from these is distinguished by being of a much smaller size.
The name of Trichocephalus has been given to nematodes which have the cephalic extremity very thin, and ending in such a fine point that it is difficult to discover the mouth. The Trichocephalus of man (Fig. 68) is a curious nematode, which was discovered by a student at Göttingen, in 1761. It is usually found in the cæcum, in which more than a thousand have been met with together. The female is from 40 to 50 millimètres long, the male about 37 millimètres. A female Trichocephalus affinis having laid her eggs in an aquarium, the whole of the contents were introduced into the stomach of a lamb, seven months afterwards, and the walls of its intestines became infested with trichocephali.
No animal at any time has attracted so much attention as that little worm which lives in flesh, rolled up; it is about the size of a millet seed, and was found by chance in the dissecting-room of a London hospital, some forty years ago. The plague and the cholera did not inspire so great fear, and this fright had almost passed from Germany throughout the rest of Europe. We were not among those who wished to take measures at all hazards against the invasion of this worm, since nothing induced us to believe that more trichinæ existed then in Belgium than in ordinary times. These measures would have produced no other effect than uselessly to disturb the minds of the public.
Trichiniasis, which was the name given to the disease caused by these worms, reminds us of tarantism, that is to say, the effects produced by the bite of the tarantula. Mons. Ozanam wrote an interesting work on this subject, in which he said that nervous tarantism existed during two centuries in Europe, as an epidemic malady. According to him, there prevails at present in the province of Tigre, in Abyssinia, a sort of chorea, or endemic musicomania, which has a great analogy with tarantism; it is the “Tigretier.” Nothing but music and dancing can have any beneficial effect during the crisis; but these means would evidently be inefficacious in trichiniasis.
The Trichina is a nematode worm, and not an insect, as it was at first called. Let us imagine an extremely slender pin, such as entomologists employ to fasten the smallest insects, rolled upon itself in a spiral form so as to lodge in a cavity hollowed out in the midst of the muscles, in a space not larger than a grain of millet. These trichinæ of the muscles can be discerned by the naked eye. But before we enter on a particular description (and they are now known in their minutest details), let us notice what were the circumstances which led to their attracting so much attention.
It was in 1832; a demonstrator of a course of anatomy at Guy’s Hospital in London, Mr. J. Hilton, found in the flesh of a man sixty-six years of age, who died of a cancer, a great number of little white bodies which he took for vesicular worms. The scalpel, during the dissection of the muscles, met with granulations which blunted the edge of the instrument. Astonished to find in the flesh hard corpuscules which the instrument divided with difficulty, he removed some of them, examined them attentively, but, no doubt, he was not sufficiently acquainted with helminthology to understand their true nature. He referred to Professor R. Owen, the celebrated naturalist of the British Museum, who recognized them as new worms, and gave them the name of Trichina, because they are as thin as a hair; he added the specific name of spiralis on account of the manner in which they were rolled up in their cyst. Trichina spiralis is therefore the name of this animal.
Some naturalists, at that time, believed that the filaments of the fecundating fluid of the male were parasitical worms, such as are found in other liquids; and these filaments which were designated by the name of spermatozoïds (the animalculæ of the older naturalists), were considered as beings having a certain affinity with trichinæ. The trichinæ were the intermediate state between these filaments of the fecundating fluid and worms properly so called. It is now known with certainty that these filamentary bodies are no more animals than the globules of blood, and that all that was thought to have been observed of their organization was nothing but pure fancy.
The trichinæ, which are now completely known in the minutest details of organization and manner of life, have a distinct mouth, and they have a complete digestive tube with an orifice at each end of the body, like all worms in the form of a thread, which, for this reason, are called by naturalists Nematodes as opposed to Cestodes (in the form of a ribbon or tape). Besides this nutritive apparatus, trichinæ, like nematodes in general, have the sexes divided into two distinct individuals, so that there are males and females, which can be easily distinguished from each other by the size and form of the body.
Trichinæ are found in the flesh of almost all the mammals. If we eat this trichinous flesh, the worms become free in the stomach as digestion goes on, and they are developed with extreme rapidity. Each female lays a prodigious number of eggs; from each of these comes a microscopic worm, which bores through the walls of the stomach or the intestines, and thousands of trichinæ lodge themselves in the flesh, where they hide till they are again introduced into another stomach. When the number is great, their presence may cause disorders or even death. Leuckart’s experiments on animals aroused the attention of physicians, and then it was found that patients who had shewn exceptional symptoms, had fallen victims to the invasion of these parasites. Leuckart counted 700,000 trichinæ in a pound of the flesh of a man, and Zeuker speaks of even five millions found in a similar quantity of human flesh.
The Trichina spiralis produces about a hundred young worms at the end of a week (viviparous); and a pig which had swallowed a pound of flesh (5,000,000 trichinæ) might contain after some days 250 millions, reckoning that only half the worms hatched were females, which is not the case, for there are more females than males. It appears that trichinæ can become sexual in all warm-blooded animals, but the number in which they can become encysted is not so great. It appears that they are not encysted in birds.
In the month of December, 1863, R. Leuckart wrote to me from Giessen; “The Trichinæ are playing a great part at present in Germany (with the exception of Schleswig-Holstein). Two epidemics have made their appearance within a few months, and have produced a veritable panic, so that no person will any longer eat pork. The authorities everywhere are obliged to subject the flesh of these animals to microscopic examination.”
We owe to Leuckart (1856 and 1857) and to Virchow (1858) the knowledge of the principal facts of the history of these worms. Virchow ascertained by experiment that they become sexual in the alimentary canals at the end of three days; and these two naturalists discovered, after many researches, that trichinæ are neither strongyli nor trichocephali, but a different kind of nematode, which are hatched in the stomach of those whom they infest, and that their embryos, instead of migrating, establish themselves in the host himself. The embryos of parasites do not usually remain in the animal which gives them lodging; they are evacuated, as well as the eggs, and are conveyed to another animal. The trichinæ are sexually developed in the same animal in which they have been engendered.
Worms which produce eggs do not usually hatch them in the same animal; they are evacuated with the feces. The trichinæ are an exception. These agamous worms, when introduced into the stomach, rapidly pass through their evolutions there, become sexual, lay eggs, and the germs which are produced from them pierce the tissues, and become encysted in the muscles or other closed organs. It appears that the Ollulanus tricuspis, a nematode of the cat, presents the same phenomena. It is a species of trichina, which lives at first in the muscles of the mouse which serves it as a vehicle, then in the stomach of the cat, where it becomes sexual and complete.
The Spiroptera obtusa is a worm remarkable for its peregrinations. It passes with the excrements of the mouse into the larva of Tenebrio molitor, which is very fond of it. At the end of a month it is encysted in this insect, and after five or six weeks it becomes sexual in the mouse. The Spiroptera obtusa of the mouse lays eggs which are evacuated with the feces; and these become, with the eggs which they enclose, the prey of meal worms, the larvæ of the Tenebrio molitor, a coleopterous insect. These germs come forth in the intestine of the larva, they perforate the intestine and become encysted in the folds of fat which surround it. Some fine day the insect is swallowed by the mouse, and the Spiroptera, set at liberty in the intestine, will be gradually matured until its sexual development is complete.
The ordinary crab of our coasts, Carcinus mænas, is the vehicle of a nematode which becomes a Coronilla robusta in the stomach of a ray.
The Heteroura androphora is another nematode which lives in the stomach of tritons. The male is always rolled round the body of its female. The two sexes are always free, contrary to that which is observed in the syngami. The Blattæ, coleopterous insects, also harbour sexual nematodes. Radkewisch saw two species of anguillulæ, the Anguillula macroura and appendiculata, in the Blatta orientalis, and an Oxyuris brachyura in the Blatta germanica. These eggs leave the body with the feces, and resist the action of deleterious agents.
Heterodera Schachtii is the name given to a nematode which Mons. Schacht discovered on beet-root. This is also a dimorphous worm; the male has the usual form, the female resembles a lemon. The Leptodera appendiculata inhabits the foot of the Arion empiricorum, in the larva state, and becomes sexual (male and female) in the decomposed body of the snail. The next generation has the sexes united, and lives in damp earth. The Leptodera pellio lives in the same way in the bodies of lumbrici; another Leptodera inhabits the intestine of the snail, and a third the salivary glands. The nematode so generally known under the name of Ascaris nigro-venosa also belongs to this genus. It lives in the lungs of the frog. There is one also in the lungs of the toad, but it differs from the preceding.
Leuckart looks upon these worms as females, and their reproduction as parthenogenetic. Schneider considers that the male exists by the side of the female sex, and that they are consequently hermaphrodites. These worms in the lungs are viviparous, and embryos are found in the midst of the intestine of the same animal which gives lodging to the female. These same worms, proceeding from an hermaphrodite parent, or from parthogenetic females, live at liberty, and not parasitically in damp earth or in a decomposed body, and differ from their parents in size as well as in sexual organs. They all become either male or female, and consequently their fecundity is dependent upon copulation. Their parents could all multiply without it, but they cannot. The females alone produce a new generation.
A worm known by the name of Vibrio anguillula lives in grains of corn while still green, and multiplies there to a prodigious extent; it is this which causes the disease known by the name of smut. The grains grow hard, and enclose nothing but little dried worms, which remain thus without apparent life, yet without dying, until they are moistened, when they become damp, the tissues swell, the organs resume their natural appearance, and the functions are restored at the end of a few hours.
In a grain of corn affected by smut, anguillulæ without distinct organs are found, which may be dried and revived eighteen times in succession, according to Mons. Duvaine, who thinks that these anguillulæ, leaving an infected grain, come out of their envelopes in a field of corn, cling to the young stalks, and rise with them. They begin to develop themselves in the rudimentary flower of the corn, and acquire genital organs like nematodes. Males and females are always found separately in a grain of corn.
The ermine lodges in its lungs and tracheal artery a long worm, to which I have given the name of Filaroides mustelarum. It usually forms a little sac, which resembles a tubercle. Many individuals of different sexes, wound round each other, are so closely tied together that they can with difficulty be separated. They resemble a ball of cotton. This filaroid sometimes gets into the frontal sinus, and mechanically destroys a part of its osseous walls, so that the skull is pierced by a hole above the frontal sinus. Dr. Weyenberg made this observation.
It is probable that other species of Mustela will present the same phenomena, for the skulls of this animal are often to be found perforated above the orbital cavity.
The Ollulanus tricuspis is a worm which lives in the walls of the stomach of cats; it is viviparous, and the young ones sometimes wander into the muscles of their host. But the natural course of things is that the young are evacuated with the feces, and that these dejecta, according to all probability, form part of the food of mice, and pass with them into the cat. It is to be hoped that Leuckart will soon put this migration out of doubt by a decisive experiment, and will prove that the mouse serves as a vehicle for three different worms, the Cysticercus, the Spiroptera obtusa, and the Ollulanus tricuspis.
Many nematodes lodge in the substance of the walls of the gizzard of birds. In the large goosander we have found one which has round its head four blades, crossing each other, toothed on the concave side. We have given the name of Ascaracantha tenuis to this worm. It has very small eggs. The Trichosomum crassicauda is a nematode of the rat; the female is 2·5 millimètres in length, and the male ·17 millimètres, and it lives in the uterus of its female. Five males are occasionally found in one female. This observation made by Leuckart has been confirmed by Bütschli. The male has its digestive tube incomplete; its female feeds for it.
The bat of the high mountains of Bavaria, known under the name of Vespertilio mystacinus, harbours a nematode, the Rictularia plagiostoma, the same which is found in Egypt in the hedgehog (Erinaceus auritus). The bat on the banks of the Rhine has not this remarkable worm. We must therefore conclude that the bat of Bavaria finds and eats the same insect as the hedgehog in Egypt, and that this insect does not live on the banks of the Rhine. We have never met with this nematode in the mystacines of Belgium, and yet we have opened them by hundreds.
A bird found in Florida, the Anhinga, has in its brain a nematode whose presence in that organ is not accidental.
The Echinorhynchi form a very remarkable group of parasites. They migrate from one host to another; but the vehicle by which the greater part of them is conveyed is not known. We represent in Fig. 72 a species which is very common in the intestine of the sprat.
It is known that these worms migrate when young, and undergo metamorphoses when they change their host. The Asellus aquaticus of fresh water, harbours besides other worms, the Echinorhynchus hœruca; the Gammarus pulex, another fresh-water crustacean, lodges the larva of the Echinorhynchus proteus (Fig. 72). We commonly find this beautiful species of the Echinorhynchus in the alimentary cavity of the sprat, and it is easily distinguished by its peculiar form and its orange colour.
The Asellus aquaticus seems also to serve as the vehicle of the Echinorhynchus angustatus. The hooks of the embryos differ from those of the adults, as the six hooks of the cestodes differ from the crown of the adults. Leuckart has described those of the envelope of the Echinorhynchus proteus and the Echinorhynchus angustatus. The embryo of the Echinorhynchus has only two large hooks on each side, but several smaller ones. The two species mentioned above have on each side five or six hooks placed at right angles with the median line, but they are not all of the same size.
The animals are allied to the Gordii in their development. In fact, their development is like that of the echinodermata; the larva is the Pluteus, in which the true echinorhynchus develops itself, borrowing the skin of the pluteus. According to the experiments made by Schneider, the larvæ of cockchafers must be the vehicles of the Echinorhynchus gigas. Pigs disseminate the eggs, and the embryos infest these larvæ, in the bodies of which they pass through their principal changes.
The Gregarinæ are microscopic beings, with an extremely simple organization, the nature and the genealogy of which have only lately been known. They live at first encysted by thousands together, under the name of Psorospermiæ; they are afterwards hatched in the form of Amœbæ, and then transformed into Gregarinæ. They migrate from one animal to another, or from one organ to another, to settle in the intestine, where they assume their adult form. In this state they are monocellular, and do not at any time possess organs which resemble the sexual organs of other classes. The disease of silk worms, known by the name of “pebrine,” has been attributed to the development of psorospermiæ.
We give the representation (Fig. 74) of gregarinæ which we have found abundantly on the Nemertes; and (Fig. 75) a peculiar species which lives in the larva of an agrion.
We also give a sketch (Fig. 76) of some very remarkable parasites, whose affinities are still problematical, and which only inhabit spongy bodies, such as the kidneys of cephalopods. The name of Dicyema has been given to them.
Prof. Ray Lankester has quite recently made some very interesting observations, at Naples, on these problematical beings; and my son has just devoted a part of his vacation, with two of his pupils, to elucidate the points of their organization and development, which are still obscure. He went to reside at Villefranche, near Nice, in order to obtain fresh cephalopods every day. His observations have led him to a result quite different from that which I expected.
CHAPTER X.
PARASITES DURING THEIR WHOLE LIFE.
In this chapter we bring together true parasites, which may be called complete; they pass every part of their life under the care of a neighbour, and require an asylum the more urgently, since they cannot exist without it. They absolutely need both food and lodging. Not long ago, all parasites were supposed to be dependant during their whole life, and to be incapable of living outside the body of another animal. We have before proved that this opinion was erroneous. We find in this category a great number of parasites which may be separated and placed in the first group, including all such as pass all the phases of their life on the same animal, without changing their costume, and many of which never leave the fur, the feathers, or the scales, among which they are born.
Fishes nourish on the surface of the skin a great number of these, which helminthologists have thought proper to classify under the name of Ectoparasites. Among many crustaceans and insects, only one of the sexes is parasitical. The males remain entirely free, and preserve all their attributes, while the females seek for assistance, and require food and lodging. The female alone sacrifices her liberty, and changes her form entirely in order to secure the preservation of her posterity.
The insects called Strepsiptera, which live as parasites on wasps, furnish a curious example of this (Fig. 77). These insects, the Polistes, the Andrenæ, and the Halicti, do not kill the larvæ of the Hymenoptera on which they feed; they suck the blood of their victim slowly, and leave him just enough strength to go through his metamorphoses. The females are condemned to remain almost completely immovable on their prey, while the males are winged.