Instead of making their attacks on those of their own class, the gadflies prefer to instal themselves on mammals and sometimes even on man. Fortunately their wants are not very great; they are contented with a little. Their presence can at most only cause some uneasiness, or some trifling functional trouble.
The œstri are dipterous like ordinary flies; but instead of passing their youth on some waste organic matter, they live in the nostrils or the stomach of some hairy animal, and undergo all their metamorphoses in the interior of its body.
Thus they pass all their youth in a crèche; but when they have reached the adult state, they get their own living in freedom.
These œstri especially attack herbivorous mammals, and the terms gastricola, cuticola, and cavicola, sufficiently indicate the places which they inhabit; the first kind lodging in the stomach, the second frequenting the skin, and the third establishing themselves in some of the cavities of the body.
Dr. Livingstone doubtless alludes to some kinds of œstri when he mentioned the numerous intestinal worms which infest animals in Southern Africa:
“All the wild animals,” says the celebrated traveller, “are subject to intestinal worms. I have observed bunches of a tape-like thread-worm and short worms of enlarged sizes in the rhinoceros. The zebras and elephants are seldom without them, and a thread-worm may often be seen under the peritoneum of these animals. Short red larvæ, which convey a stinging sensation to the hand, are seen clustering round the trachea of this animal, at the back of the throat; others are seen in the frontal sinus of antelopes; and curious flat leech-like worms are found in the stomachs of leches” (a new species of antelope).[4]
A species, peculiar to the horse in Europe, usually lives in its stomach in summer; and when its development is complete, the winged insect follows the course of the food, and goes out from the anus to breathe the open air. The mother fly, excited by the sentiment of maternity, flies round the breast of the first horse that she meets, and lays her eggs there on some hairs which are not beyond reach of the animal’s tongue. The horse wishing to get rid of these foreign bodies, licks them off, and thus they are introduced into the mouth, and from the tongue pass to the stomach. These eggs are hatched in the midst of the gastric juice, the larvæ leave them, and the young gadflies find in the juices of the stomach the milk which serves to nourish them. These larvæ pass through their metamorphoses in the stomach, and when the young fly has assumed its perfect form, with its delicate wings, its sucker, and its facetted eyes, it leaves the stomach, follows the path traced by the food, arrives some fine day at the rectum, presents itself at the place of exit, and takes its flight. Thus the fly can take its journey through the intestines on a portion of the digested food.
When she has once taken her flight she is very near the end of her life, and after a moment of love she gives up her place to others.
There is another gadfly which finds a crèche in the sheep; but instead of lodging in its stomach, it instals itself in the nostrils, which are more easily reached. This second species goes through its evolutions in the vestibule.
This is the species which sometimes introduces itself into the body of man. Many instances of this have been known, and our late colleague Spring gave a very interesting account of one of them in the bulletins of the Belgian Academy.
A gadfly found at Cayenne is distinguished by the name of the Macaco Worm; it belongs to the genus Cuterebra, and usually attacks the skin of oxen and dogs in South America. It is accidentally found sometimes on man. This is the Cuterebra noxialis. We here give the representation of it.
There is also a gadfly on the ox.
Professor Joly has devoted himself to zoological researches on Œstridæ in general. Professor Schroeder Vander Kolken, in Holland, and Mons. Brauer, in Austria, have studied them with great success.
The Hippoboscus is a fly which is very greedy of blood, and attaches itself to horses and oxen, especially under the tail, in the parts where there is less hair. It sometimes also attacks man.
The Hippoboscus lives on the horse, and an allied species, of which a different genus has been formed, lives on bats (Strebla vespertilionis) in South America. Mons. Von Baër noticed hippobosci on the elan, during his residence in Königsberg.
Many other insects live and develop themselves at the expense of their nearest neighbours.
Travellers since Azara’s time assure us that Uruguay contains but few oxen and horses, because a fly exists in that country which lays its eggs in the navel of these animals at the moment of their birth. These animals, on the contrary, are abundant in Paraguay. In order to increase their number in Uruguay, it would be necessary to favour the multiplication of birds or insects which make war on these flies, either in the larval or the sexual state.
Diptera, known by the name of Conops, pass their first three changes in the soft parts of drone-bees. Dumeril had formerly suspected, from the curvature of the abdomen, that the Conops lays its eggs in the body of some other insect. Lachat and Victor Audouin have given an instance of this in the “Journal de Physique.”
Thus the Conops, in its larval state, inhabits the abdomen of drones or other hymenoptera; the Echinomyæ are developed within various epidoptera when in the state of caterpillars or chrysalids; there are even some which live on flesh, and prefer that which is in a state of incipient putrefaction.
We may also speak, in this category, of animals which seek assistance, while young, from neighbours of whom they take advantage during their life, and utilize them even after their death; these are insects of various orders. They are in general more cruel than beasts of prey, which often contend on equal terms with their victims. Here we have an enemy which furtively introduces itself into its neighbour, who is nearly sucked dry before he suspects the danger to which he is exposed. He harbours unawares the assassin who is about to murder him. This is the refinement of cruelty.
The Melophagus of the sheep is a wingless dipterous insect, like the Lipoptena of the stag. We give figures of these two curious insects.
The Stratiome chameleon pays visits to flowers to seek for insects, on whose blood it feeds. Its very elongated larva lives in stagnant water.
We have now to mention in the following passages parasites much less cruel in general, and which receive with greater delicacy the hospitality which is afforded them. We refer to some worms which pass, not their youth, but their mature age in the body of a neighbour, and use their host not as a crèche, but as a lying-in hospital.
Their early youth is passed in freedom, but they soon give birth to a numerous progeny. The fate of the male is unknown; as to the female, she introduces herself in a microscopic state into the body of a neighbour, is developed there till she arrives at sexual maturity, and then quits her retreat to go and scatter her eggs.
It appears, however, that these females are obliged to seek assistance from insects; but before they enter this living asylum, the male, which is not yet known, ensures by his fecundation the preservation of the species.
We often find in summer in puddles of water, thin worms, which are sometimes a foot long, resembling a violin string, and have for a long time puzzled naturalists. They are known by the name of Gordius, and have lately been very carefully studied, both with reference to their organization, to their mode of life, and their development. We give here the figure of a Gordius of the natural size. The Mermis, like the Gordius, passes its youth in the body of certain insects, and leaves its living cradle to scatter its eggs abroad. In this case, the embryos themselves go to seek for their host, and unlike the ichneumons, they use them with moderation. The life of the host is never compromised, and no functional disturbance is observed, notwithstanding the enormous size of the worm.
The Mermis is especially found after a heavy shower; some kinds of Filaria are also more common when it rains. Under the title of “Notes on the Appearance of Worms after a Shower of Rain,” I communicated to the Academy of Belgium some observations on these creatures, and these observations were recorded in the bulletins.
Some years ago they brought me one morning, after a shower of rain, a quantity of worms, four or five inches in length, very thin, and twisted round each other, which had been collected in the morning, on the flower borders of several gardens within the city. It was thought that there had been a shower of worms in the night.
There was not one male worm among three hundred; all were full of eggs, and the young ones were already wriggling about within them.
Whence come they? said I, in my article. Have they fallen from the sky completely formed? It is evident that they have not been developed on the ground where they have been found; it is not less evident that they appeared suddenly on the borders. Did they come from within the bodies of certain insects which they have quitted, on account of the rain which had fallen? These worms, in fact, had completed their parasitical stage in the bodies of their hosts, and the great drought which had continued for many weeks prevented their resuming their first course of existence. It was the sudden emancipation of so many worms at once which had attracted the attention of gardeners: earwigs, cockchafers, and many other insects give them shelter during the time of this strange gestation.
It is known, by the observations of Siebold, that the eggs of the Mermis, laid during the winter, produce in the following spring embryos which live in damp earth. They immediately seek the larvæ of insects, perforate their skin, and develop themselves there without becoming encysted. After this, they again pass through the skin of their host, return to the damp earth, where they change their skin, are fecundated, and lay eggs. The larvæ of Mermis albicans especially resort to caterpillars, or the larvæ of the coleoptera, orthoptera, or diptera, and even to a mollusc, the Succinea amphibia.
Professor Meissner, and more especially Dr. Grenacher, professor at Göttingen, have made known to us the structure of the Gordius. The Gordius bifurcus produces embryos at the end of a month; these embryos perforate their shell by means of their beak, become free in the damp earth, and introduce themselves through the skin into the perigastric cavity of certain larvæ. The sexual worm again becomes free. If we may believe Mons. Villot, who has made recent observations on the Mermis and the Gordius, the latter alone pass through complete metamorphoses; they assume three different forms, and change their habitation three times. Their first abode must be in the water, or in the larva of a dipterous insect, as a free embryo; the second in the larval state, in the intestines of a fish; and the third, like the first, in a sexual state.
To judge by some specimens of gordius brought from India, these curious parasites exist not in Europe only; they have been found in different parts of the world, and they lead everywhere the same kind of life.
They have been found in Calcutta in the Hapale; in the Philippine Islands in a Mantis, and the museum of Hamburg possesses some from Venezuela, which came from the body of a Blatta.
These worms, when they approach the adult and sexual age, lose their various external organs, and are so completely modified with respect to their organization, that at last they are merely a case for eggs. They are so entirely egg-cases, in which the digestive tube and the other organs disappear in proportion as the sexual organs are developed, that many naturalists have taken these worms for a simple ovisac. This has also been the case with the Nematobothrium of the fish known under the name of the eagle-fish; it has been taken by an eminent naturalist for a nest of psorospermiæ.
There are also worms which take refuge in plants, and live at their expense, as if they were in an insect. One of the most remarkable is that which attacks corn, and produces the disease known by the name of smut, the corn eel (Anguillulina tritici). It is a very small and thin cylindrical worm, which dries up completely with the grain of corn which has nourished it, and which can remain for an indefinite period without dying, in a state resembling dust. Every time that it is moistened, it resumes its activity. This return to life has been compared to a kind of resurrection.
Mons. Davaine has studied this worm with great care; he has made known the different phases of its development, and the manner in which it introduces itself into the plant and the grain. Needham, in his “New Discoveries made with the Microscope,” (1747) gives a whole chapter to these microscopic eels.
The larvæ of the Anguillula scandens are dried in the galls inhabited by the mother. As soon as these galls fall and grow moist, the larvæ revive, and abandon their cradle to live in freedom. Soon after this, they go in search of their plant, take it by storm, and penetrate into the tissues before the period of fecundation; having become sexual in the interval, these microscopic nematodes lay their eggs in a nest formed at the expense of the plant.
Another species lives in the dipsacus, in which also it produces disease (Anguillulina dipsaci). It attacks the flowers, and remains on them without signs of life till the moment that they are moistened. The vinegar eel is another nematode worm which has some affinity with the preceding ones. It has been considered a Rachitis.
There exists also a river species; but have not different worms been confounded under this name? Many species live in brackish water, and these are remarkable for the presence of bristles on their heads, and by very distinct eyes.
[3] The discovery of a free bothriocephalus at the bottom of a ditch caused a great sensation in the world of naturalists some years ago. It was then thought that the parasite could not exist except in the body of an animal: they could only imagine it shut up as in the cells of a gaol.
[4] Missionary Travels in South Africa, p. 136.
CHAPTER IX.
PARASITES THAT UNDERGO TRANSMIGRATIONS AND METAMORPHOSES.
A certain number of parasites establish themselves at first in an animal which serves as a crèche, then in a second which serves as a lying-in hospital. This passage from one animal to another is described under the name of transmigration. In general, the entire crèche with its nurslings passes into the lying-in asylum. The crèche is always represented by an animal which feeds on vegetable diet, which is destined for one which is carnivorous: the lying-in asylum is represented by the latter. The mouse is the crèche which will pass with all its clients into the cat which eats it.
If we were treating of plants, we should say that in the first host they are developed, and in the second they blossom. The plant, like the animal, is agamous as long as the flower and the sexual organs have not made their appearance.
The animal which migrates usually undergoes a complete change in passing from one abode to another; it is agamous in the first instance, that is to say, without sex, swathed and covered with a padded cap like a nursling; in its last stage it is, on the contrary, endued with all its sexual attributes.
In the crèche the parasite is on its passage from one station to another, and that which arrives at the lying-in asylum has reached the end of its journey and is at home. We have proposed to give it the name of Nostosite, as distinguished from that which only inhabits its host for a time. We may also remark that the same animal may give lodging to these two kinds of parasites. It is thus that the rabbit harbours in its peritoneum passengers which are only at home in the dog; and, independently of these passengers (these strangers may we say?), it lodges in its intestines a sexual tænoid worm. The first is a Xenosite, the second a Nostosite. The mouse, in the same manner, gives lodging to passengers under the name of Cysticerci, which are destined to the cat in order to become Tæniæ.
We might call the rabbit or the mouse which harbours worms in transitu, the stage coach; more especially as from time to time there are some which miss it, and are consequently lost in their peregrinations.
This stage-coach is the intermediate host, the Zwischenwirth of German helminthologists, which is always an animal with a vegetable diet; the final host is generally a carnivore: it is by means of the vegetable feeder, the grazing or herbivorous animal, that the stranger parasite introduces itself.
The result of this is, that the carnivore receives into its house, every time that it devours its prey, all the parasitical inmates of the latter, and the walls of its digestive canal form the soil in which are implanted all the worms which can take root there. The tissues of the prey are triturated and digested, but the worms which it encloses escape the action of the gastric juice, and are set at liberty in the stomach. The stomach of the carnivorous animal is a sieve through which thousands of parasites are often introduced at each repast, and fishes lodge many which often pass from one stomach to another. Their whole life is spent in these migrations; they are travellers who have their abode in railway carriages, and never take their departure at the stations.
Each stomach is, in fact, a station, very frequently quite filled with merchandise, which disappears with the station itself by the next train. Happy are those who find themselves in a carriage safely on the rails towards its destination. Many are called but few chosen. How many journeys some of these travellers have to take before they find their host!
It is often very interesting to open a fish which has made a good meal; its stomach and intestines contain, first of all, the usual worms; the half-digested prey, in its turn, encloses some; and it is not rare to find besides them the parasites of those which were swallowed together with their host.
The animal is usually attacked in its youth by the parasites which it harbours all its life. In order to know the inhabitants of some fishes, we must examine them shortly after they are hatched.
In the crèche the parasite occupies an organ which is closed, and without communication with the outer world; it inhabits the garret of its first host; in its last host, which represents the maternity asylum, it dwells, on the contrary, in the largest apartments, and never ceases to be in direct communication with the exterior. Thus, in the first animal, it is often completely immovable and under a form which we have named scolex; in the latter it moves freely, and has, in addition to sexual organs, those which are proper to this condition which we have called Proglottis. Thus these parasites undergo metamorphoses.
For a long time, metamorphoses seemed to be the attributes of frogs and insects exclusively. In the class of worms, in which they are complicated with the change of hosts, they much surpass in reality the most brilliant and extravagant fictions of the poets. The phenomena of these transmigrations were completely unknown before our researches were made. If some naturalists, like Abildgaard or Pallas, suspected their existence, it was rather by accident, and the experiments to which they devoted themselves were all unfavourable to their suppositions.
The knowledge of these transmigrations has at the same time dispersed the latest illusions of the partisans of spontaneous generation; it was the more difficult to explain the presence of worms in enclosed organs, since these worms were always without sex. By the same means, we have ascertained the true prophylactic treatment, and thus discountenanced the numerous anthelminthic remedies which had often caused more serious accidents than the parasites themselves.
When it was considered that parasites were the result of an especial degeneration of some of the intestinal papillæ, the physician would at once consider that there was some morbid condition, and we can understand that all his efforts would be employed against the enemy which had arisen. Now it is known that every healthy animal living in freedom contains parasites almost as invariably as the organs which support its life; and it is not a matter of doubt to us that parasites often play their allotted part in the economy; their absence as well as their presence may be the cause of inconvenience. We should not even be astonished if the administration of certain worms internally should be prescribed as a remedy. Have we not known the time when all maladies were supposed to yield to the action of leeches, and do we not see the good effects of their application? There are many kinds of parasites, and their therapeutic effect may, perhaps, in future, form an interesting subject of study.
To speak at the present time of a verminous temperament would be scientific heresy, an anachronism; this shows the progress that we have made of late years. Valenciennes was permitted to employ this language at the Academy of Sciences in Paris not twenty years ago, and Lamarck wrote thus in his standard work on invertebrate animals, in the beginning of this century: “It is very certain that there exist in a great many animals, and even in man, intestinal worms; some of which are formed there, others are born and all live there, multiplying more or less, without any of these worms showing themselves externally, or being able to live elsewhere.
“During so many centuries that observations have been made, well-ascertained species of intestinal worms have been found nowhere else than in the bodies of animals. We are now authorized to believe that there are innate worms, or such as are produced by spontaneous generation, and that these are modified from time to time; this is at present the opinion of the most enlightened observers.”
Thus it was considered by Lamarck that parasitical worms are only found in the bodies of animals, and are actually produced there.
Can it be believed that such ideas were put forward by zoologists of the highest merit? and ought we to feel surprised that the theory of spontaneous generation was so long taught in the physiological schools?
A book published in 1859 was entitled, “Heterogenesis, or a Treatise on Spontaneous Generation.” The author gives the clue to the origin of his errors in the second line of his preface, in which he says: “When, by meditation, it was evident to me that spontaneous generation was one of the means employed by matter for the reproduction of living beings.”... According to this philosopher, science is, therefore, not the generalization of facts, but these facts must serve to prop up the theories or hypotheses invented in the silence of the study. This passage of his work shows us that he was no more able to yield to the evidence of experiments made on worms, than to those of Pasteur on the infusoria.
It may be related to the honour of the illustrious Baer, that, from the year 1817, during his stay at Königsberg, he took up arms against this hypothesis, and never ceased to combat it, till evidence succeeded in opening the eyes of the most obstinate.
The worms which present the most remarkable phenomena of transformations, accompanied by metamorphoses, are the Distomians and Cestodes, flat worms, which we will consider in the first place.
Trematode worms include a certain number of large and beautiful parasites which scarcely undergo any change, and are found only on the skin and the gills of certain fishes; these are the monogenetic trematodes, comprising the Tristomidæ and all the worms of that group, which also stand higher in their organization: we shall speak of them hereafter. The other trematodes, which are called digenetic, live on the most dissimilar animals, under the most varied forms, and, like the greater part of the cestodes, introduce themselves into the individual who is to give them shelter, only by the assistance of a host, acting as a stage-coach which serves them as a vehicle.
The principal family is that of the Distomidæ, a family par excellence cosmopolitan; as inconstant in their progress as capricious in the choice of their companions. Each distome resembles a small leech which has a sucker in the centre of the belly, and as this sucker was once considered to be perforated, the name of Distoma was given to them.
These parasites are the more interesting to us, from the fact that, though we are not the final resting-place of certain species, we nevertheless find them pass through us on their way. There are two species which occasionally lodge in the liver of man without being peculiar to him, for they properly belong to the sheep. Two other distomes have lately been described by Dr. Bilharz, which are fortunately only known at present in Cairo, and which are interesting, both with respect to their organization and to their manner of life.
The genealogy of the distomidæ is now generally well known; that which remains to be discovered is the itinerary of each particular species; and in several zoological laboratories experiments are daily made with certain species and the hosts which they are supposed to seek. These investigations have already yielded the best results in the laboratories of Giessen and of Leipzic, under the direction of Leuckart.
The genealogy of the distomidæ is as follows: the young distome, when it leaves the egg, is wrapped in a ciliated tunic, and, under the guise of a microscopic infusorial, it abandons itself to all the vagaries of a free and vagabond life; this is the bright period of its life. “It is a youth starting, with all the steam up, without help and without guidance, in the midst of the ocean; if it meets an island on its passage, that is to say, the body of an aquatic larva or a mollusc, it disembarks, brings forth its young, and disappears; its purpose is fulfilled. If it find no island or continent it sinks and perishes, for it carries no provisions with it; it has no organ which permits it to take nourishment on its passage.” If life is short, even in the case of a young distome, it is passed in the midst of the water: if fortune is favourable to it, it will at last meet with a living abode, where it will find all that is necessary to the comfort of a parasite.
Abundance always reigns in these living oases; and as these new colonists are really exiles, who will never again see their native country, ciliary oars are useless to them, and their descendants differ entirely from their common mother.
Under the ciliated tunic of the mother appears a daughter under the form of a bag, who is born almost at the same time as herself, and concerning whom we may quote here the words of Réaumur: “Singular and mysterious duality in unity; two beings, living one within the other, which are still only a single individual. Has nature accustomed us to such profusion? Do we ever see her retrograde thus from a more complicated organization to one more simple?” That which this great observer did not dare to believe has yet been realized, and in many cases development is clearly recurrent.
Led by a marvellous instinct, and obeying an irrevocable mission, the distomidæ, as well as the monostomidæ, and others besides them, when they claim an asylum from molluscs, introduce into the living body of their new host, not an isolated embryo, but a young animal already impregnated with a rich posterity; if she remain mistress of the situation, this posterity will forcibly invade the various organs, without any consideration whether their host may not give way under the weight of this sudden invasion.
Fig. 41.—Monostomum verrucosum, Sporocyst with Cercariæ. In front is the mouth, in the middle the digestive canal, and around the digestive canal are young ones, under the form of Cercariæ in process of development.
Fig. 41 represents one of these worms which proceeds from a ciliated embryo, and encloses by the side of its digestive tube cercariæ in different degrees of development. In front, we see one provided with eyes and a tail; behind, we see others which are younger; among these ciliated embryos, wandering without guidance and without a compass in the midst of their ocean, but few will reach the land, or, in other words, will find the port where their progeny may prosper. This first embryonic state is that in which there are the greatest perils. When stripped of their swimming tunic, these young distomes have the form of a bag, which for a long time was called a sporocyst. From these sporocysts we see hundreds and thousands of young ones proceed, resembling in no respect the mother which has brought them into the world. These, in their turn, will resume a free and independent life. They are colonists whom the distome has left on a foreign land. This simple multiplication is often not sufficient for the preservation of the species; the first sporocyst produces other similar sporocysts, and these bring into the world a rich progeny of tadpoles, which after a certain metamorphosis will become sexual distomes. These tadpoles are often well armed, and devour occasionally even the last scrap of flesh belonging to their host. They have long been known under the name of Cercariæ, which was given to them at a time when their genealogy was unknown. They are not very unlike the tadpoles of the frog (Fig. 45). The mother was only a bag with ciliæ, and sometimes with eyes. The tadpole has a distinct body, with a movable deciduous tail; and after this falls off they have sexual organs.
The cercariæ often abandon their first host in which they have been developed, and live at liberty in the water while waiting for their final host. They are taken sometimes in the open sea. In 1849, J. Müller wrote to me from Marseilles that he had just discovered cercariæ and distomes living at liberty in the Mediterranean. Since then this illustrious naturalist has observed them again at Trieste, while pursuing his studies on the Echinodermata, and has had the kindness to send me his original drawings of these singular parasites.
We have found both at Marseilles and at Trieste, says J. Müller, a new cercaria with a pinnate tail, and two black ocular points; its body is from one-tenth to one-sixth of a line in length, not including the tail, which is twice or two-and-a-half times as long. There is a protuberance just in front of the middle of the body. At each side of the tail there are from twelve to twenty pencils of soft bristles placed on little prominences in a transverse series of six tufts, not regularly opposed to each other. In one specimen, the tail, from its point of insertion to the posterior quarter, is provided with these bundles of bristles; and in another they are wanting entirely in the anterior half, but exist, on the contrary, on the hinder half. In a third, the bristles have partially disappeared, and are reduced to six bundles at the extremity of the tail. This tail presents traces, more or less distinct, of transverse rings. J. Müller has often seen that the distome, which proceeds from this cercaria, swims freely in the sea, and after having got rid of its tail, could be easily recognized by the two black marks which were then more diffused.
This cercaria described by J. Müller recalls to us that which was noticed by Nitzsch on fresh-water shells (Cercaria major) with an annulate and pinnated tail.
Claparède also took at Saint-Vaast, cercariæ the host of which he did not know. This naturalist supposed that this worm could migrate at will. He found there the same cercaria (C. Haimeana) on Sarsiæ and Oceaniæ, but always sexless.
The Cercaria setifera of J. Müller has been found free and attached to the lower surface of some medusæ. It exists occasionally in considerable numbers on the internal surface of some Acalephæ of the ocean and of the Mediterranean. Claparède has also observed another free cercaria which bears the name of Pachycerca.
Some of the cercariæ are very tenacious of life; we have kept some alive in fresh water during a whole week in the month of November, and on the last day they were still active (Cercaria armata). We sometimes find the cercarian age passed over, and the young distomes appear abundantly without tails in the sporocyst. We have seen an example of this in the Buccinum undatum of our coasts. This latter generation assumes in every case a very different form from that which preceded it.
Lodged and nourished without expense in the succulent parenchyma of their victim, the cercariæ grow rapidly, and as soon as their caudal oar is developed, they tear asunder the membrane which encloses them, and abandon their host in order to live freely as tadpoles. Some fine day, tired of their nomadic life, they choose another host, get rid of their tail, fold themselves up in a winding-sheet, like a chrysalis about to become a butterfly, and concealed in a sac, which is designated by the name of cyst, they wait patiently for days, weeks, or years till their host is swallowed by the creature intended to lodge them. The cyst is set free in the stomach of the latter host, its envelopes are dissolved in the juice secreted by its enclosing membrane, and with its whole establishment the worm recovers its liberty in this new abode.
The encysted cercariæ pass thus with arms and baggage into the stomach of a new host. Their envelopes, not to say their swaddling-clothes, are torn to pieces by the gastric juice, and at the end of their stage they go and lodge in larger apartments, more appropriate to their new wants. The time of their celibacy is passed, and a numerous progeny, under the form of eggs, is prepared. In this condition they fulfil their last mission; and if their mother, the sporocyst, knew only the joys of agamous maternity, the cercaria which has just become a distome appreciates all the sweetness of sexual maternity.
The distome thus reaches the termination of its voyage and of its evolutions; it lays its eggs in the midst of the feces of its host, and millions of animalculæ watch for the new brood, while others wait for the visit of the ciliated generations. The daughter distome thus differs completely from her mother sporocyst, but she resembles her grandmother who has lived in the same manner as herself. Thus we have animals free and vagabond when they leave the egg, and which swim vigorously like infusoria without depending on others. But the end of their life approaches, they strip themselves of their ciliated mantle, and being again closely swathed up before they die, they seek the hospitality of a mollusc and give birth to their numerous progeny.
We have therefore animals whose little ones in swaddling clothes live at first at liberty, and seek for assistance when the moment for thinking of a family approaches. The descendants lead, like their parents, a wandering life; and as their mother threw off her ciliated cloak, so they abandon their oar-like tail, to think in their turn of family cares.
To sum up all, there are in the life circle of a distomian two distinct forms, which begin and end in the same manner, the first putting forth a progeny by means of buds, the second by eggs. There is alternation of form, on account of the double multiplication (digenesis) and migration through several individuals. In other words, the young distome, before it reaches its destination, must change its train many times, and it wears in each carriage a different costume. We can easily understand how difficult it is to recognize this travelling distomian, as it changes continually its railway-train and its dress, and what sagacity must have been employed by naturalists in order not to lose its track.
We may give more than one description of the distomian embryo as it leaves its sporocyst. Is it a mother and an enclosed daughter, as is the case with aphides, or is the ciliated envelope merely a cloak? We think that the latter is the true interpretation. The ciliated mantle which the embryo loses, is a skin which has been thrown off in moulting, a simple effect of age.
Thus we find in the complete evolution of a distome an organic and a sexual age, a true alternation; the agamous age undergoes a true moulting, the sexual age a metamorphosis.
We have before considered the embryo as mother and daughter coming into the world together, as we see among the aphides; or the mother, daughter, and granddaughter are born together like twins; so that if the mother or the daughter meet with an accident during parturition, the granddaughter may be born before her mother, and even before her grandmother.
We are now about to study some of these mysterious travellers which have given so much trouble to naturalists to discover their abode and determine their identity. Considering the number of observers who have mentioned these distomes, it is evident that these parasites must be very common. We find the names of Ruysch, Leeuwenhoek, Swammerdam, Camper, Houttuyn, Mulder, Heide, Biddloo, Snellen, etc., among the naturalists who have made them a subject of study. In our own day, the writers who have explored this territory are so numerous that we should require more than a page simply to give their names.
Distomes frequent, with few exceptions, all the classes of the animal kingdom, and if their number is great among fishes, they are not less numerous in mammals and birds. The higher classes of animals usually inoculate themselves through the intermediation of molluscs, worms, and crustaceans, and it is therefore in the ranks of these that we must seek for their first abode. Without admitting that their size bears some proportion to the host which gives them shelter, still, the largest species, the Distomum Goliath, is found in the liver of one of the balænoptera. This distome is of the size of a large leech, and its host does not measure less than twenty metres.
Mons. Willemoes-Suhm mentions a distome which at the time of its cercarian evolution lives freely in the water, and attaches itself by its sucker to the larvæ of worms or copepod crustaceans, and then lodges in their dejecta without encysting itself. This is the Distomum ocreatum of the herring, according to Professor Moebius. Mons. Ulialnin found in the bay of Naples another free distome, which is also attached by its ventral sucker to certain copepods, and which becomes the Distomum ventricosum inhabiting many kinds of fish.
Any one who wishes to make observations on distomes in the state of cercariæ has only to examine some fresh-water molluscs, either the Limneæ or Planorbes found in ponds; as he tears the animal to pieces on the stage of a simple microscope, he will not fail to perceive a multitude of struggling and wriggling tadpoles. Their tails twist with each other, furl up, extend, and describe arcs of circles, as if we had a nest of serpents under our eyes.
Each species of distome has it own cercariæ, which are scattered among as many different inferior animals. Birds and fishes become infested by them in consequence of eating these animals.
Fig. 42.—Liver fluke of twice the natural size; a, mouth; b, penis; c, digestive tube; d, abdominal sucker.
We may here cite as an example of this class of parasites the Distomum hepaticum, or liver fluke; this species is the most interesting to us of all the genus; it attains the size of a moderate leech, and habitually resides in the liver of the sheep. In order to discover it, we have only to examine a fresh liver. They are usually found in the biliary canals, where they move about like planariæ. It is always of a deep colour, and is doubtless introduced in the state of cercaria, when the animal is drinking. M. Willemoes-Suhm supposes that the Distomum hepaticum has for a vehicle a small snail, the Limax agrestis, which the sheep swallows with the grass on which it feeds. Its principal abode is in the ruminants and only casually in man. It is said to be unknown in Iceland. The Distomum lanceolatum has also been found in man.
Dr. Bilharz, the pupil of Siebold, discovered in the year 1851, on man, a parasite in every respect remarkable. It belongs to the family of the Distomidæ, and on account of its peculiarities, it has been made into a genus under the name of Bilharzia. It is found in Egypt, and lives in the vena portæ and in all its ramifications in man. According to Bilharz, this distomian is diœcious, the male being of considerable size, the female slender and delicate, which fact does not agree with the usual characteristics of diœcious animals. At least half of the Fellahs and Copts suffer from these parasites; these worms, at the period when they lay their eggs, proceed from the vena cava to the veins of the pelvis, and after having produced very grave consequences, they are at last evacuated with the urine.
Another distome was also found by Bilharz in the intestines of a young Egyptian boy.
The largest known distome inhabits the liver of the Balenoptera rostrata, the little whale of thirty feet in length, which is regularly met with on the coast of Norway. The intestines of the ordinary seal often contain a very curious distome, which was first observed by Rudolphie, the D. acanthoides. The seal is also infested by the Distomum cornus, which some have incorrectly preferred to place in the genus Amphistoma.
Besides the distomes which inhabit the liver, there are found but few in the mammalia, except in the Cheiroptera: these insectivorous animals have their intestines literally full of these parasites. We have noticed the species which regularly frequent our bats, and it only remains to discover the insects by means of which they are introduced; for it is probable that these insects are infested by cercariæ during the time that they inhabit the water. Larvæ and their parasites ought to be carefully studied in the localities where bats abound.
There are few birds, especially among the grallæ and the palmipedes, which do not enclose in their intestines a certain number of distomes. The same may almost be said of reptiles and batrachians, but it is especially in fishes that their number is greatly increased. We may say that there is no fish which does not nourish some of these trematodes. Among a portion of these, the cycle of evolution and transmigration is perfectly known; we may instance the Distomum nodulosum. This worm inhabits the intestines of the perch.
The scolex, as well as the cercaria, has its particular characters, and we have long since found the latter in a fresh-water mollusc, the Paludina impura. The cercaria is easily recognized by the presence of two particular folds at the base of the buccal bulb, and by the transparency and the form of the extremity of the urinary apparatus. In the adult distome, this same part of the urinary apparatus encloses large vesicles with very distinct partitions.
We may also mention among the distomes a species from fish, which has a great affinity with the singular distome observed by Bilharz, of which we have spoken above. This distome inhabits the “castagnole,” or Brama raii. Under the opercula of this fish, the skin is folded, and forms one or more pouches, in each of which lives a coupled distome, that is to say, by the side of each large and fat individual, full of eggs, there is one which is slender. It is the Distomum filicolle, to which the name of Monostomum was at first given. We should be correct in supposing that of these two hermaphrodite worms one acts rather as a female, the other as a male. It is doubtless in this sense that Steenstrup maintained his assertion, that there are in nature no hermaphrodites.
Thus there are two kinds of distomes: the first live in couples in a cyst, the second in couples joined together, but at liberty; and in each case only one individual produces eggs. These are distomes which act really like diœcious worms. We find, however, a more remarkable instance in the Monostomum bijugum of Miescher. In the tumours which are formed in the beak of the grosbeak (Fringilla), he has constantly found two individuals; and in many cases he has surprised them with the penis of one engaged in the sexual organ of its companion. These worms, while they live in couples, resemble each other like snails and leeches; they are mutually fecundated, and both lay eggs.
Leuckart recognized these sexual distomes in their cyst, in the larvæ of ephemerides; and Linstow noticed a distome thus sexual and encysted in the Gammarus pulex.
The name of Monostoma has been given to some of these trematodes which have no abdominal sucker.
One of the most curious worms of this group is the Monostomum mutabile. It lives in the sub-orbitary sinus of several aquatic birds; that is to say, in the nasal fossæ, especially of water-rails and moorhens. We give a slightly magnified representation of them. It is a worm resembling an elongated leaf. By compressing it slightly on the stage of the microscope, we easily discover the ovary, the matrix, and oviduct full of eggs. By isolating some of the eggs, and crushing them gently to break the shell, we set free the worm (Fig. 44), quite different from the mother (Fig. 43). The former has two eyes surrounded by a ciliated mantle, and by means of this ciliated envelope, the monostome swims freely in the water. If we compress it slightly, we see that in the interior of the ciliated covering, there is still another animal, without eyes, without ciliæ, and of an entirely different form, which in its turn encloses a whole progeny.