Chapter IX: 183 (1)

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 184 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 185 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, 186 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 187 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 188 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 189 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 190 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 191 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 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 192 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 193 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 194 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, 195 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 196 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 197 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 198 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.

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 199 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 dioecious, the male being of considerable size, the female slender and delicate, which fact does not agree with the usual characteristics of dioecious 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 200 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, 201 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 dioecious 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 202 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.

The embryo, having long ciliæ in front, and in the interior a sporocyst already full of young cercariæ, is shown in Fig. 44. It is this latter creature which the ciliated embryo must confide to the care of others; this she puts out to nurse with some mollusc or other, until it is fit to provide for itself in its turn. We have still to discover the train by which the parasite must travel, in order to 203 arrive again at the nasal fossæ which are the first cradle of the family.

We find occasionally between the feathers of some birds tubercles of the size of a pea, and when we open them we see in each two similar worms, placed so that the stomach of one is applied to that of the other; this is the monostome of which we have spoken above. These worms are from three to four millimètres in length (about ·13 in.), and are found in the titmouse, the siskin, the sparrow, the canary, and some other birds.

A worm very common in the intestines of the green frog is known by the name of _Amphistomum sub-clavatum_. Its cercariæ are usually found in an acephalous mollusc, known by the name of _Cyclas cornea_. That which distinguishes the scolices of this species is the great contractibility of the external membranes of the young individuals; they lengthen, they shorten, they swing to the right and the left, describing a semicircle on the anterior half of the body (Fig. 46). We represent side by side the cercaria of this amphistome, and the adult 204 and sexual amphistome, as it is found in the intestines of the frog.

Constantine Blumberg has recently published an interesting memoir on the structure of the _Amphistomum conicum_.

A beautiful trematode worm, known by the name of _Hemistomum alatum_, whose antecedents have not been ascertained, lives usually in the intestines of the fox. It is about four or five millimètres in length (about ·17 in.). Many birds harbour Holostomes which belong to the same group, the first state of which is not yet known. The _Holostomum macrocephalum_ is common in the intestines of rapacious birds; it is from five to seven millimètres in length (about ·23 of an inch).

We close the history of trematode worms by giving the figure of a beautiful one known under the name of _Polystomum_, which lives in its adult state in the bladders of frogs (Fig. 48). Interesting observations have recently been made on the manner in which they are introduced into the bladder.

The worms which naturalists call Cestoïds, or Cestodes (which means, like ribbon or tape), have for their type the tape-worm known by every one. They are very abundant in many animals, are found in almost every class of the animal kingdom, and are almost as common as the distomians, of which we have just spoken. They are introduced into animals which are vegetable-feeders, by means of water and plants, and into carnivorous animals by their prey. The tape-worms of the herbivora lay eggs like the others, but their embryos have, as soon as they are hatched, a ciliary covering which allows them to live and 205 move about in the water. Those of beasts of prey are entirely different; it is by means of the prey that they enter their hosts. Each carnivore has its own worms, as it has its own prey which introduces them.

Independently of these worms, the vegetable-feeders afford lodging to some which are not their own.

We have found in bats two tæniæ, both incompletely developed, and 206 occupying the digestive tube. One has a rostellum without hooks, like the tæniæ of the vegetable-feeders, the other has hooks like those of the carnivora. These cestode parasites are observed to be of two principal forms; the first vesicular, like the finger of a glove partly drawn inwards. They are always lodged in the midst of the flesh, or in a closed organ in the middle of a cyst; under this form the cestode worm is harboured by a host which is to serve as a vehicle to introduce him into his final host. He is a parasite on a journey; he is always agamous, and usually bears the name of cysticercus (Fig. 49). As to the second form, it is like a ribbon; it attains a great length, always occupies the intestine, attains its complete and sexual development, and lays an innumerable quantity of eggs which are disseminated with the evacuations.

The rabbit harbours a cysticercus which has its final destination in the dog (a xenosite); but independently of this stranger, it gives hospitality to a special tænia in its intestines. This is its own worm, the _Tænia pectinata_, which is a nostosite. All the herbivora are in a similar case; the ox and the sheep possess a peculiar tænia of their own, besides those which they lodge for the sake of the 207 carnivora. The worms of the herbivora have particular characters by which they are easily known; they have no crown of hooks.

The tænia of the wolf, which has often been confounded with the _Tænia serrata_, lives in the brain of the sheep, and produces a disease known as the "gid." It was formerly said that every animal has its enemy. We should rather say that each species has its parasites, and each parasite has its vehicle by which it is introduced.

These tape-worms are found in all the vertebrate classes. An herbivorous animal usually serves as a vehicle, but it more frequently carries, besides its passengers, species which are peculiar to itself. As the carnivorous animal is not intended to be eaten like the herbivora, it cannot serve as a vehicle, and if by chance its muscles enclose some passenger, he has lost his way and that for ever.

Do the cetacea generally live on fish, and do they become the prey of some aquatic carnivora? We have reason to think so, from the presence of certain agamous cestodes, which have been frequently found in too great number to allow us to suppose that they have lost their way in these aquatic mammals. There have been seen in the substance of the muscles of many species, or rather in the layer of blubber which covers the skin, agamous cestode worms of the genus _Phyllobothrium_, which can only accomplish their evolution in some large _squalus_. There must then be contests between dolphins and sharks, contests in which the dolphins are worsted, in spite of their superiority. These Phyllobothria have been found in the _Delphinus delphis_, the 208 _Tursio_, and the _Ziphius_. As the _Orca_ attacks the whale, and feeds upon its flesh, there would be nothing surprising in our finding in these large cetacea, some agamous cestode destined to pass through the last phase of its evolution in this terrible carnivorous animal.

The cestode can scarcely be called a parasite under the first vesicular form. It is sufficient for it to pass through its first transformation in the midst of the tissues, and it will remain weeks, months, even years, without undergoing any change; it asks for nothing but an hospitable roof; and this mysterious being, that had often come they knew not whence, encamping rather than lodging, always without progeny, was long since cited by the naturalists of a former age in favour of the old hypothesis of spontaneous generation.

It is not the same with the second form. Here the worm, always lodged in the intestines, grows with extraordinary rapidity, and fulfils all the conditions of a true parasite. In a fertile soil it extends itself and produces young as long as it has any life, and in no group of the animal kingdom do we find any fecundity to be compared to that of this worm. Boerhaave described a broad tape-worm, three hundred ells in length. Eschricht estimates the number of the segments of this worm as ten thousand; and if we consider that each segment, or, we should rather say, each complete worm, may perhaps enclose thousands of eggs, we may form some idea of the profusion of germs which can be scattered by each individual.

To thoroughly know an animal we must have made observations on it during all the phases of its evolution. Let us sketch these phases. 209 All the cestodes have eggs, usually in great number, very well protected against external agents. They endure heat and cold, drought as well as humidity, resist by means of their envelopes the most violent chemical agents, preserve the faculty of germinating, we will not say for weeks, months, and years, but for centuries. When they first leave the egg, we see an embryo of an oval form, transparent, composed apparently of sarcode, contractile throughout all its extent, and in the middle of which we perceive six stylets arranged in pairs, and which at last move with great rapidity.

The following is the manner in which, some years since, we described these six hooked embryos produced by a tænia of the frog, which were struggling by the side of each other on the slide of a microscope. "The six hooks are arranged regularly in each individual, and move exactly in the same manner. They are very slight, and of nearly half the diameter of the embryo. Two occupy the median line, and unite like a single stylet; these are nearly straight, and a little longer than the others. They only move backwards and forwards. Their action is like that of the parts of the mouth in certain parasitical crustaceans, the Arguli, when they endeavour to pierce through the tissues. They are in continual motion to and fro. The other four hooks are similar to each other, and differ from the first in the point, which is curved into real hooks. They are arranged two and two, to the right and left of the first, so that they all meet at the base. Their movements are not the same as those of the two first; they remain almost fixed at the base, while they describe a quarter of a circle at the extremity. Let us imagine the six hooks, placed in front in the 210 same direction. The two in the centre advance, and the two pairs placed symmetrically by the side of them, are lowered and drawn backwards, and thus push the body forwards.

"It is like the dial-plate of a clock, with three hands placed by the side of each other; that in the middle would advance directly forward, while the two others would be lowered until they formed a right angle with the first. This is the movement which we observe in all the stylets. The result of this is that we distinctly see the embryo penetrate between the _débris_, or into the crushed tissues which surround it. These embryos imitate the movements of a man who wishes to get through a window a little above him, and who, having succeeded in passing his elbows through, pushes his body forward by leaning them on the frame.

"We see the same efforts continue for hours; and we can easily understand that there is no living tissue, however dense it might be, except the bones, which could not be easily penetrated by these microscopic embryos. This explains why we so commonly find cysticerci scattered in cysts along the intestines and between the membranes of the mesentery, and how they can, by piercing the walls of the vessels, spread themselves into the most distant organs, by means of the blood which conveys them. When the embryos have once pierced these walls, they hollow out the tissues in all directions, until they find themselves in the muscles, or in the organ which is indicated in their itinerary. When they have arrived at their destination, they stop and surround themselves with a sheath; their stylets, which are no 211 longer of use to them, decay; and at one of the extremities appears a crown of new hooks quite different from the former ones, which will serve to anchor their progeny in the new host into which they may be introduced."

Thus the vesicular worm (Fig. 50), fully formed, and without undergoing any change, waits till its host, or the organ which shelters it, is eaten, and then wakes up in the stomach. Every living cysticercus which penetrates into the stomach, instantly quits its torpid state: it gets rid of its useless parts, abandons its former cavity, penetrates into the intestine, attaches itself by its new hooks and its suckers to the enclosing membranes, and grows with such rapidity, that in less than six weeks, we often find a tape-worm many metres in length. The vesicle which had hitherto protected it, is abandoned, and the part which remains with hooks and sucker is the mother which has produced in this agamous manner the whole colony. This mother is usually called the head of the tænia, or more properly the scolex. As long as the mother is there, she engenders and produces cucumerinæ, that is to say, proglottides, which are the perfect and sexual state of the cestode.

We have seen among the trematodes a worm of a particular form leave the egg, and immediately produce a swarm of young ones, which go and live separately. In the cestodes all these individuals are united 212 in a kind of band, and are besides this joined to the mother, which becomes the root of the family. This root, planted in the walls of the intestine, is the head. Thus each segment of the tænia is an individual, and at the period of sexual maturity, this individual is detached, goes away with the feces, spreads over the grass or elsewhere, and thus sows far and wide the eggs which it contains.

The tænia, as well as the other tape-worms, is generally looked upon as an imprisoned parasite during the whole of its existence. This is a mistake; the last stage of the life of cestodes is a phase of liberty. The cucumerina, or, as we have proposed to call it, the proglottis, that is to say, the complete and sexual animal, is evacuated with the feces; and when we notice a dog leaving his dung upon the grass, it is not uncommon to see there worms which move like leeches, and whose white colour is in strong contrast with the mass which contains them. The duration of this last stage is very short, it is true; but it is, nevertheless, during this period of her life that the mother scatters the eggs which are to disseminate the species.

We repeat that each animal has its parasites, and these in their turn are not always exempt from them. We have already cited some examples of this.

Man has the dental system of a vegetable feeder; but, thanks to fire, which he alone knows how to produce and maintain, he eats flesh. It is by these means that he nourishes the solitary worm, which, by its crown of hooks, is a cestode belonging to the carnivora, and the _Tænia mediocanellata_ with the _Botriocephalus_, which are cestodes 213 peculiar to vegetable-feeders. As a feeder on vegetable diet he also harbours vesicular agamous cestodes, which are only found in him as passengers.

The _Tænia serrata_ of the dog lives at first as a passenger in the peritoneum of the hare and the rabbit; and every one knows how greedily the dogs eat the viscera of these animals.

The cat entertains another kind of tænia, and, as we may easily suppose, in its young state it lives as a passenger in the mouse or the rat. Who then has traced out for it this itinerary, and pointed out the way, the only one by which the parasite can hope to take possession of its proper abode? Evidently it is neither the tape-worm nor the cat. The plan for all these various species is marked out beforehand, and each animal as soon as it is born knows it without being taught.

A Danish naturalist, Mons. H. Krabbe, has just finished a special work on cestode worms of the genus _Tænia_, and he remarks that there is no class in which these worms are so abundant as in that of birds. It is among the rapacious and carnivorous birds of this class that they are less abundant. Among mammals, the carnivora possess the greater number. This fact, as M. Krabbe remarks very rightly, seems to indicate that the cestodes of birds especially employ the inferior aquatic animals as their vehicles when in their incomplete state.

Let us consider the solitary worm of man (_Tænia solium_), it will enable us to understand all the others. Known by the name of tænia, or solitary worm, it is, like all the cestodes, a marvellous association of mothers and daughters, which are developed and vegetate in a 214 peaceable community. Each segment is a complete being, which encloses within itself an entire and very complicated apparatus for the fabrication of eggs.

We give (Figs. 51 and 52) the representation of a solitary worm, peculiar to man, of the natural size; and at the side the scolex, usually called the head, slightly magnified.

Under its first vesicular form the solitary worm is planted in a 215 provisional soil. After this it is transplanted into a richer soil, where it flowers and throws out its numerous seeds. It comes to us from the flesh of the pig, in which there lived vesicular worms, of the size of a hazel-nut. The muscles are sometimes full of them, and the pig is then said to be "measly." The ancients noticed that the sucking-pig never takes this disease; and as _Sus scropha_ is the name of the pig, the term scrophula has the same origin as the specific name proposed by Linnæus.

The measles in pork have been attributed to damp, to feeding on acorns, to hereditary causes, to contagion, even to injured corn and mouldy bread. All these theories we find in pathological treatises. The only true cause, however, is the introduction of the eggs of the _Tænia solium_ into the intestines. If we wish to prevent this infection, we must not permit the animal to eat man's excrements, nor to drink water in which substances that have become decomposed on a dung-heap have been allowed to remain.

The cysticercus of the pig, when introduced into man, becomes a tænia with as great certainty as the seed of a carrot will produce this plant if sowed in suitable soil. The observation had been for a long time made without any explanation being given, that this parasite especially shows itself among pork butchers and cooks. This is because these persons, more frequently than others, handle raw pork. The same observation has been made respecting children who have made use of the gravy of raw meat. Minced raw meat (_conserve de Damas_) has been prescribed with success in chronic diarrhoea. The tape-worm has often been known to make its appearance after this treatment, as 216 may well be supposed. Tænia helminthosis is constant and general in Abyssinia, and they there commonly eat raw beef. Those who do not eat meat, as the monks of certain orders there, who live only on fish and flour, never have the tænia. Ruppell and many others have noticed this fact. Mons. Küchenmeister says that at Nordhausen, in the Hartz, as well as throughout all Thuringia, measles are very prevalent among pigs; and as the people are in the habit of eating minced pork, both raw and cooked, spread on bread for breakfast, this country may be looked upon as the Abyssinia of the north.

The doctor at Zittau caused a man who was condemned to death, to take, seventy-two hours before his execution, some cellular cysticerci from a measled pig; and he found in the duodenum of the man four young tæniæ, and six others in the water in which they had washed the intestines. The latter had no hooks, but those of the former had some in every respect similar to those of the _Tænia solium_.

We have ourselves caused a pig to swallow eggs of the tænia, and have given it the measles. Messrs. Küchenmeister and Haubner, who were ordered by the government of Saxony to make some experiments, also caused three pigs to swallow eggs of the _Tænia solium_, and two of these were affected with measles. A piece of flesh, weighing 4-1/2 drams, contained 133 cysticerci, which amounts, for 22 German lbs., to 88,000 cysticerci.

The use of raw pork will produce tæniæ more readily than raw beef. Dr. Mesbach has given the following instance in support of this fact. At Dresden, a father and his children regularly ate, at their second 217 breakfast, raw beef, but one day they took pork instead, and eight weeks afterward one of the children, when in the bath, voided two ells of _Tænia solium_.

The etiology and prophylaxis of the solitary worm, that is to say, its mode of introduction, and the means of protecting ourselves from it, are clearly indicated. It is sufficient to introduce one of these vesicles into the stomach in order to have the tape-worm. The experiment has been made: young men have ventured, in the interests of science, to swallow some, and have ascertained how many days were required for the parasite to be sufficiently complete to give off segments with the feces.