CHAPTER V. 85

PARASITES.

"En plongeant si bas dans la vie, je croyais y rencontrer les _fatalités physiques_, et j'y trouve la justice, l'immortalité, l'espérance."--MICHELET, _l'Insecte_.

The parasite is he whose profession it is to live at the expense of his neighbour, and whose only employment consists in taking advantage of him, but prudently, so as not to endanger his life. He is a pauper who needs help, lest he should die on the public highway, but who practises the precept--not to kill the fowl in order to get the eggs. It is at once seen that he is essentially different from the messmate who is simply a companion at table. The beast of prey kills its victim in order to feed upon his flesh, the parasite does not kill; on the contrary he profits by all the advantages enjoyed by the host on whom he thrusts his presence.

The limits which separate the animals of prey from the parasite are usually very clearly marked; yet the larva of the ichneumon, which eats its nurse, piece after piece, resembles a carnivorous animal as much as a parasite. There are indeed certain animals which take advantage of the good condition of their Amphitryon, but which 86 render to him in return precious services. Thus those which live on the produce of the secretions, or which clear the system of useless materials in exchange for the hospitality which they receive, are not true parasites. These services are of a very different character, and the duties which they sometimes perform for each other are in some respects analogous to medical care.

Every animal has its own parasites, which always come from without. With some few exceptions, they are introduced by means of food or drink. In order to ascertain their origin, the naturalist must beforehand study the food, that is to say, the prey or the plant which furnishes the habitual nourishment of the host which gives them shelter.

A carnivorous animal, however, does not in general content himself with a single kind of prey--one voracious animal of this class devours all that comes in its way; another, more of an epicure than a glutton, chooses with more discernment. But in the midst of this varied kind of food there is always some species which forms the staple of the usual bill of fare, and it is necessary to find out what this is if we wish to ascertain the parentage and the metamorphoses of the parasite, since it is that which conducts the parasite to its new destination. The mouse is destined to the cat, and the rabbit to the dog; in the same manner, each one of the herbivora is intended to be the prey of a carnivorous animal, if not larger and stronger than itself, at least more cunning. It is of great importance to discover the animal which conducts the new-comer into his habitation. When we know it, we have only to introduce into it the stranger guest, that sooner or later 87 he may pass into the body of his accustomed Amphitryon. In order thoroughly to know these sedentary or vagabond populations, we must not only study them at the different periods of the year, and under all the conditions of their irregular life, but it is necessary to follow them from the moment that they quit the egg till their complete evolution, closely noticing all that relates to their reproduction.

In the dung of the cow, by the side of the elegant _Pilobolus_, live masses of small eels, born in the stomach of the animal, which wind and twist like microscopical serpents, and do not seek the slightest help from the organ which shelters them. They are hatched in the interior of the stomach, as if it took place in the meadow. These little eels have evidently only the appearance of parasites, and it may be that they render some service in some of the organs through which they pass. This may also be the case with those which live on the feces of others, or which, lodged in the rectum, watch for the prey which is attracted by the odour. These, especially the latter, are rather messmates than parasites. True parasites are animals entirely dependent on their neighbours, unable to provide for themselves, fed entirely at the expense of others. It is generally supposed that parasites are exceptional beings, requiring a place by themselves in the animal hierarchy, and knowing nothing of the world except the organ which shelters them. This is an error. There are few animals, however sedentary they may be, which are not wanderers at some period of their lives, and it is not even uncommon to find some which live alternately as noblemen or as beggars. Many of them only deserve to be placed among paupers when they are in their infancy or 88 at the approach of adult age, for they only seek for help at the beginning or towards the end of their career. These are very numerous, and more than one species change their dress so completely that they can no longer be recognized. Finding with their host both food and lodging, they throw off their fishing and travelling gear, settle themselves comfortably in the organs which they have chosen, and having got rid of the baggage which connected them with the outer world, preserve only their sexual organs.

As to the rank which these parasites occupy in the scale of being, it may be said that there is no especial class of parasites; and worms are not distinguished in this respect, except by having a greater number of species subject to this rule. All classes among invertebrate animals include parasites.

It is also an error to suppose that the whole species, the young as well as the old, the males as well as the females, are always parasites; often the female, not being able to provide for the necessities of life, seeks for food and shelter, while the male continues his nomad life. Therefore the female alone puts on the pauper's dress, and by a recurrent development, assumes sometimes such a singular appearance that the male no longer resembles her. One cannot say that the females constitute the _beau sexe_ in this group, since they are often so monstrous in form and size that their appearance has nothing in common with a perfect animal; their body is deprived of all its exterior organs, and there often remains only a skin in the form of a leather bag, without any distinguishing character.

What is still more astonishing, is to meet with males which, under the 89 conditions to which we have just alluded, come at last to seek for assistance from their own female, so that she has to provide for all; and the charitable animal which comes to her help takes the whole family under his charge. Assistance is thus thoroughly organized in the lower world; neighbours are found which serve as a _crèche_ for the indigent when they first quit the egg, others as a hospital for the infirm adults or the females, and others again play the part of innkeepers for all, instead of affording a place of refuge for some privileged individuals.

There are but few animals, if indeed there are any, which have not their peculiar parasites. Of all the fishes of our coasts we have never found but one which had none; and perhaps, could we observe this fish in different latitudes, we might find that it had its poor dependants as well as the rest.

Thus we may assume that no animal is free in this respect, and man himself regularly affords hospitality to many of them. We feed some with our blood and our flesh; there are some which lodge on the surface of our skin, others in the interior of our organs; some prefer to establish themselves on children, others on adults. The name alone of some is sufficient to make us shudder, while others live peaceably in some crypt, without our suspecting their presence. Who is there that does not nourish some acari, of the genus _Simonea_, in the membrane of the nose? In fact, man gives a home to some dozens of parasites, and the presence of the most terrible among them constitutes, in certain countries, a condition of health which is envied. The Abyssinians do not consider themselves in good health, 90 except when they nourish one or many tape-worms.

Among the animals to which man gives his involuntary assistance, we may mention first, four different Cestoidea, or tape-worms, which live in the intestines; three or four Distoma, which lodge in the liver, the intestines, or the blood; nine or ten Nematodes, which inhabit the digestive passages or the flesh. There are also some young Cestodes, named _Cysticerci_, _Echinococci_, _Hydatids_, or _Acephalocysts_, which find in him a _crèche_ to shelter them during their life. These always choose enclosed organs, like the eye-ball, the lobes of the brain, the heart, or the connective tissue. We also provide a living for three or four kinds of lice, for a bug, for a flea, and two ascarides, without mentioning certain inferior organisms which lurk in the tartar of the teeth, or in the secretions of the mucous membrane.

There are some animals which harbour few inhabitants, while there are others that keep up a great retinue; and it is not always, as we have already said, that those who give lodging to but few enjoy the most excellent health. We might give as an instance of this, a fish which is known to all, the turbot, which as well as the woodcock is highly prized, though both have their intestines literally obstructed by tape-worms and their eggs. We have never opened one, large or small, lean or fat, which had not its intestines filled with cestode worms. They are so numerous as to form a kind of cork, which one might think intended to close the passage of the pylorus.

Some authors give remarkable instances of the abundance of parasites. Nathusius speaks of a black stork, which lodged twenty-four _Filariæ 91 lobatæ_ in its lungs, sixteen _Syngami tracheales_ in the tracheal artery, besides more than a hundred _Spiropteræ alatæ_ within the membranes of the stomach, several hundreds of the _Holostomum excavatum_ in the smaller intestine, a hundred of the _Distoma ferox_ in the large intestine, twenty-two of the _Distoma hians_ in the oesophagus, and a _Distoma echinatum_ in the small intestine. In spite of this affluence of lodgers the bird did not appear to be in the least inconvenienced.

Krause, of Belgrade, mentions a horse two years old, which contained more than five hundred _Ascarides megalocephalæ_, one hundred and ninety _Oxyures curvulæ_, two hundred and fourteen _Strongyli armati_, several millions of _Strongyli tetracanthi_, sixty-nine _Tæniæ perfoliatæ_, two hundred and eighty-seven _Filariæ papillosæ_, and six _Cysticerci_. When we consider how many eggs a single worm produces, we can understand how it is that so few animals escape being invaded by them.

Sixty millions of eggs have been counted in a single nematode, and in a single tape-worm, or rather in a colony, even a thousand millions of eggs. Even the very animals which live as parasites, harbour others in their turn. We find parasites on parasites, as we find messmates upon messmates. Almost all writers on this subject give examples of these; some in the larvæ of ichneumons, others in the lernæans, and we have more than once met with nematodes in different crustacea still attached to their host.

In order to understand thoroughly the living furniture of an animal, especially of a fish, it is necessary to examine it while young; the feces are the _Kitchen-middings_ of the stomach; it is from them 92 that we can appreciate the bill of fare of each. This study of the food will one day excite much interest, not only in a scientific point of view, but also with reference to fishing as an occupation.

There are some animals which are infested at every period of their life, and at every season; others in far greater number only during their youth, and they gather in at the commencement of their life the harvest for the rest of their days. The greater part of parasites, especially of fish, are introduced with the first nourishment. As soon as they issue from the egg, young rays, like young turbots, are already stuffed with worms which afterward obstruct the digestive organs. The stomach of each of these fishes is like a filter which allows every thing which is food to pass, but detains on its passage and without any change all that is living. When we examine the stomach and observe the food in its different degrees of digestion, we see distinctly the worms coming out of their holes, wallowing in that which physiologists call chyle, and choosing afterwards at their convenience the place where they may completely develop themselves. At the end of a few days, the fish may have swallowed an innumerable quantity of small animals, and if each of them introduces some worms, we can easily understand in how short a time the intestine becomes literally filled.

There is no organ which is sheltered from the invasion of parasites: neither the brain, the ear, the eye, the heart, the blood, the lungs, the spinal marrow, the nerves, the muscles, or even the bones. Cysticerci have been found in the interior of the lobes of the brain, in the eye-ball, in the heart, and in the substance of the bones, as 93 well as in the spinal marrow. Each kind of worm has also its favourite place, and if it has not the chance of getting there, in order to undergo its changes, it will perish rather than emigrate to a situation which is not peculiar to it. One kind of worm inhabits the digestive passages, some at the entrance, others at the place of exit; another occupies the fossæ of the nose; a third the liver, or the kidneys.

We may even divide parasites into two great categories, according to the organs which they choose: those which inhabit a temporary host, almost always instal themselves in a closed organ--in the muscles, the heart, or the lobes of the brain; those, on the contrary, which have arrived at their destination, and which, unlike the preceding, have a family, occupy the stomach with its dependencies, the digestive passages, the lungs, the nasal fossæ, the kidneys, in a word, all the organs which are in direct communication with the exterior, in order to leave a place of issue for their progeny. The young ones are never enclosed. Even the blood is not free from these animals, but there are few which lodge there, except during the act of migration.

In Egypt, Dr. Bilharz observed a distome in the blood of a man (_Distoma hoematobium_); the _Strongylus_ of the horse has been long known, which causes serious injuries in its vessels (_Strongylus armatus_); as also the strongylus of the dolphin and of the porpoise (_Strongylus inflexus_), and the filaria of the dog (_Filaria papillosa_); and some are also found in the blood of many birds, of reptiles, batrachians, and fishes; so that there is no class of vertebrates which escapes.

There are some which, like leeches, seek assistance from their 94 neighbours, but are content to snatch their food as they pass, and only attach themselves for a short time to the host which they despoil; they retain their fishing or hunting tackle, as well as their organs of locomotion. These parasites, which never take up their lodging on the host which nourishes them, have no sooner sucked his blood, or devoured his flesh, than they resume their independent life.

They do not disfigure themselves, nor put on any special costume, like those which seek a permanent abode. Gluttony is not with them the only moving principle of existence; they do not forget what they owe to the world, and keep up an appearance which allows them at all times to present themselves afresh.

Parasites are scattered over every region of the globe; they choose their place, and observe, like all living creatures, the laws of geographical distribution. All do not inhabit the animal kingdom; some seek for assistance in vegetable life. Many insects lay their eggs in seeds or fruits, and their progeny, as soon as they are hatched, find abundant nourishment in the sap or in the farina stored up for the young plant; others pass into a state of lethargy while the seed is dry, and recover their activity every time that they receive a little humidity.

The female of a coleopterous insect deposits its eggs in the nut, and in proportion as this grows, the young larva devours the kernel. When it is brought to table, it encloses only the skin and the excretions of the larva. A weevil establishes itself in a similar manner in cereal plants, and, small as it is, it may produce great calamity by multiplying in granaries. There are even worms which lodge in 95 certain of the graminaceæ, and get completely dry with the envelope which contains them, without ceasing to live. Their life is suspended till the day when the seed is sufficiently softened in the earth or the water.

We have seen that each parasite has its host: we must have a particular name to designate it. But that does not imply that if it find not its dwelling-place it must perish. It may only live some time at the expense of its neighbour, and thus pass for its parasite. Naturalists are occasionally deceived. Thus, they once believed in the passage of the Schistocephalus of the stickleback into the intestines of certain birds which eat them, and in which they are only found accidentally. The Ligulæ of the Cyprinidæ, found in the intestines of the cormorant or the goosander, are not, in our opinion at least, worms peculiar to these birds. They are strangers which must either emigrate again or die. Acari which originally belonged to mammals and birds, have been found living on man, causing prurigo, or even serious maladies, and yet these parasites are not regarded as peculiar to our species. We might cite other examples. Who has not been annoyed by the flea, which abandons for an instant the dog, its natural host?

Among these free parasites, many do not attach themselves to a particular species, and well deserve the title of cosmopolitan parasites. Thus we see that the _Ascaris lumbricoides_, so common among children, lodges also in the ox, or the horse, the ass, and the pig. The _Distoma hepaticum_, which is a parasite peculiar to the sheep, if we may judge by its abundance in this animal, may find its 96 way into the liver of man, or into that of the hare, the rabbit, the horse, the squirrel, the ass, the pig, the ox, the stag, the roebuck, and different species of antelope. It is to be remarked that all these animals have a vegetable regimen. By drinking the water which contains the cercaria of this species, they grow infested by this singular lodger. The large Echinorhyncus (_E. Gigas_) has been found in the dog, and the pig, perhaps in the phocinæ; and instances are mentioned in which it has even migrated into man. The _Gordius aquaticus_ appears to live and develop itself in different species of insects; and among the articulated parasites, we meet with the _Ixodes ricinus_, commonly called the tick, on the dog, the sheep, the roebuck, and the hedgehog; and instances are given of its presence on man. It has been long since proved in menageries and zoological gardens, that the _Acarus_ of the camel is able to give a cutaneous disease to man.

As we have before said, there are many parasites which require to be studied in order to determine the host peculiar to each of them; although parasites sometimes lose their way, and introduce themselves into the wrong neighbour, yet they can live there but a short time. Instances have been known, in which the larvæ of flies have penetrated into man accidentally by the mouth or the nostrils. Reptiles have been known to live a certain time in the stomach. A German physiologist, Berthold, professor at the University of Göttingen, has given an account of all those which have been found under such circumstances, and the number of them is considerable; he has written a memoir on the abode of living reptiles in man.

Among other instances, this naturalist mentions the case of a boy of 97 twelve years of age, who, in 1699, after suffering acute pain, voided from the intestines nearly one hundred and sixty four millipedes, four scolopendræ, two living butterflies, two worm-like ants, thirty-two brown caterpillars of different sizes, and a coleopterous insect. These animals lived from three to twelve days. This is not all: the same child, two months afterwards, voided four frogs, then several toads, and twenty-one lizards, and sometimes a live serpent was seen for a moment at the bottom of his mouth. Happily for science, we do not see such things seriously related in books at the present day.

The size of parasites is very various: Boerhaave mentions a bothriocephalus three hundred ells in length; at the Academy of Copenhagen, it was reported that a solitary tape-worm (_Tænia solium_) had been found eight hundred ells long. Female strongyli have been seen from two decimètres to one metre in length; and _Gordii_ of two hundred and seventy millimètres. We have found in a fish a worm which lived rolled up like a ball, and which measured, when unrolled, more than a mètre.

Parasites present an extraordinary variety of forms, and the differences between the sexes in size as well as in appearance are greater than in any other group of animals. The male of the _Uropitrus paradoxus_, the Urubu of Brazil, has the usual form of a round long worm, while the female resembles a ball of cotton, without the slightest analogy with the other worms of the order. The Lernæans also have females excessively various in size and appearance, while the males generally resemble each other in their external 98 characters. What is not less remarkable is, that hermaphrodite worms often unite in couples, and that only one of the two seems to perform the function of a female, and increases in size (_Distoma Okenii, Bilhartzia_). It even happens that the union is so complete that the species appears formed of two individuals fastened to each other. The Diplozoa show us a curious example of this. The gills of breams are usually infested by these last-mentioned worms. Nothing is more strange than to see all these individuals united two and two as if soldered together, each preserving its mouth and digestive canal, and producing eggs which give birth to isolated individuals. We sometimes see males so completely absorbed in their females, even in an anatomical point of view, that they only represent a fragmentary apparatus. The male of the _Syngami_ is so obliterated, that when compared with the other males of its order it is only a testicle living on the female.

Should an organ infested with worms be considered diseased, simply on account of their presence? We hesitate not to say that, as long as these guests cause no disorders, there is no pathological condition. The child which has _Ascarides lumbricoides_ in its stomach is not necessarily ill. All animals in a wild state always have their parasites; they lose them rapidly when in captivity.

The Abyssinians do not take medicine when they have tæniæ; on the contrary they are in a better state of health. Do we not find medical men prescribing the employment of leeches, and consequently calling in the assistance of certain parasitical animals? This action, far from 99 being a cause of sickness, is in this instance a remedy, and no one can foresee all that science has a right to expect from the salutary effects of certain parasitical worms on the system. There are, if we mistake not, many discoveries in store for observers in this order of investigation.

But here, as in all things, excess is hurtful. Certain organisms, developing themselves immoderately, may break the harmony necessary between the parasites and the host which they frequent. It has been found recently that many morbid affections, as the potato and vine diseases, have for their origin only the abnormal development of certain microscopic beings hidden in the organism.

It is found, that in Egypt, a distoma is developed in the blood, and occasions a very severe malady, scarcely known to physicians. In Iceland, a cestode causes the death of a third part of the population. Worms develop themselves in the eye, and may even cause blindness; the _Coenurus_ of the sheep causes giddiness, and becomes fatal to the animal which harbours it. The chlorosis observed in Egypt and Brazil must, it appears, be attributed to a considerable development of a nematode worm, which lives in the small intestines, and which naturalists know under the name of _Dochmius duodenalis_; and lately the Trichinæ set all Europe in a state of excitement, and trichinosis was for a time more dreaded than cholera. In spite of all these accidental circumstances we think that the animal which possesses its ordinary parasites, far from being ill, is in a normal physiological condition.

When we consider these animal parasites in general, one would think 100 that their tenacity of life is very feeble, and that the slightest derangement would be sufficient to kill them. It is not so; on the contrary, some of them can be entirely dried up, and return to life every time that they are moistened; and the eggs of some of them resist the most violent reagents. We have known eggs preserved for years in alcohol, in chromic acid, and in other agents which destroy life everywhere else; and then give birth to embryos directly they are placed in pure water or damp earth.

Some years ago they had no idea of the migration of animals from one body to another. As we have said elsewhere, Abildgard, half a century ago, made experiments on the worms of fishes which he caused ducks to swallow, but these experiments had no result, and formed rather an obstacle to ulterior progress, than an approach to truth. The worms of fishes have been known to live in birds; but these worms were only there as adventitious parasites. Liguli live some days in the goosander, but they do not maintain their position.

Our great initiator into the world of parasites, Mons. Siebold, arrived also at a conclusion which could not be maintained. Having observed, with his habitual sagacity, that the cysticercus of the mouse is the same worm which lives in the cat, he published his opinion that the eggs of this tænia had lost their way in the mouse, that the young worms had become sick there, and that in the cat alone, they could be healthily and completely developed. It was like a plant lost on a soil where it could not live, and still less flourish. May I be permitted to state by what means we have arrived at the knowledge of the transmigration of worms?

I had commenced the study of encysted Tetrarhynchi in the peritoneum 101 of the Gadidæ in 1837. Ten years afterwards, shortly after a visit from my learned friend, Mons. Kölliker, I discovered that this world of parasites did not live such a monotonous life as was supposed. I ascertained by my dissections of fishes, that the tetrarhynchi also, which were supposed to be disinherited by Nature, knew how to vary their pleasures; that instead of spending their whole life in a prison cell, they change their home at a certain age, and pass the latter part of their existence in more spacious habitations.

I had seen the _Tetrarhynchus agamus_ inhabiting a cyst in the peritoneum of the gadidæ, and I had met with the same tetrarhynchus completely developed and sexual in the spiral intestine of the voracious fishes known under the name of squalidæ, or sharks. This caused me to write to the Academy of Brussels, at the meeting on January the 13th, 1849, that the order of vesicular worms, admitted by all helminthologists, ought to be suppressed.

These worms began to be understood when these cysticerci ceased to be regarded as sick creatures. Siebold had mistaken the _crèche_ for the hospital, and instead of seeing in the cysticercus a young animal full of life and of the future, he looked upon it as a gouty individual, ready to breathe its last sigh.

These fish had directed me in the right road; I had closely followed up certain very characteristic worms, which lived under a very simple form in certain fishes, and which, passing with their host into the stomach of another, finished in the latter their toilet and their evolution. I had been a witness of all their changes of form from 102 the cradle to the tomb, by following them from fish to fish, or rather from stomach to stomach. In fact these parasites are perpetually on their journey, and constantly changing their host, and at the same time their dress and mode of locomotion, so that frequently, at the end of their voyage, they preserve only shapeless rags to cover their eggs or their offspring.

That which adds still more to the difficulty of recognizing them is, that while young they are often enveloped in swaddling clothes which nevertheless permit them to wander freely; then in a simple robe, in keeping with the home which shelters them; and at last in a wedding dress, which hides the eggs and the apparatus which produces them. The nymph in her virgin condition has none of the attributes of future maternity.

It is in this category that we find the Distomes, so common in all the classes of the animal kingdom. This is not all: frequently, among these various forms, these animals when young produce little ones, which in no respects resemble the others, and are not even formed in the same manner. As soon as they quit their swaddling-clothes, they increase by gemmation, and without sexual union, while those which are produced from buds increase sexually. Thus the daughter does not resemble her mother, but her grandmother. This phenomenon has been known by the name of alternate generation; we have called it _digenesis_.

But all parasites do not resemble those distomes, which change several times both their host and their costume. We find some of them, which the mother deposits with care in the body of a neighbour, and which pass all their early life in the viscera of an alien mother. Such are the Ichneumons, beautiful winged insects, which perfidiously 103 insert their eggs in the body of a living caterpillar, whose internal part serves at the same time for a cradle and for food. The young larva devours organ after organ, beginning with the least important, till the last serves for the formation of the last members of the winged insect.

More unfortunate are those which are kept under the bolts and bars of their host from their early youth to mature age; they have no participation in the great banquet of life, except it be in the pleasures of the table and of love. We also find some parasites which occupy different organs in the same animal, and which have different sexual attributes according to the situation which they inhabit. We know some which are hermaphrodite in the rectum or in damp earth, and whose young ones, having the sexes separate, live as parasites in the lungs.

Parasites are not usually reproductive in the animal which they inhabit. They respect the hearth which shelters them, and their progeny are not developed by their side. The eggs are expelled with the feces, and sown at a distance for other hosts.

Parasites may be divided into several categories. We may bring together in the first of these, a certain number of animals, which, without being true parasites, seek for a place of shelter, and, either on account of their wretchedness or their misery, require this protection in order that they may live.

In the second category, we may place those which live at complete liberty, and only require for their sustenance the superfluities of their neighbours; they take great care of the skin of their host, 104 and use it sparingly. Some also are found which cannot live without assistance, but repay it with some service. Often, indeed, they associate with their host, and live on a footing of perfect equality with him; and besides these are found associations in which equality is by no means recognized, and where labourers or even slaves perform the work disdained by their masters.

In the last category we shall arrange true parasites, which take both their lodging and their food. And here, again, we shall meet with three distinct subdivisions.

The first includes those which travel from one hotel to another before they arrive at their destination; to-day they lodge in a prawn, to-morrow in a gudgeon, then in some fish which preys upon others, as the perch or the pike. These are nomadic parasites, which do not stop or think of family life until they have found the hotel for which they are destined.

Sometimes the parasite gets into a wrong train, and not being able to retrace his steps, he remains at a station where no other train will take him up. He is condemned to die in a waiting-room.

In the last subdivision, we have parasites that have arrived at their destination, occupying themselves in future only with the joys of a family.

Thus we find some which are really at home, and others which are on their journey, sometimes on the right road, and at others, wandering and lost in an alien "host." The former are _autochthonic_ parasites, the others are foreigners. We may say that each animal species has its proper parasites, which can live only in animals which have at least more or less affinity with their peculiar host. Thus 105 the _Ascaris mystax_, the guest of the domestic cat, lives in different species of _Felis_, while the fox, so nearly resembling in appearance the wolf and the dog, never entertains the _Tænia serrata_, so common in the latter animal.

The same host does not always harbour the same worms in the different regions of the globe which it inhabits. This relates both to the parasites of man, and to those of the domestic animals. Thus the large tapeworm of man, which naturalists call _Bothriocephalus_, is found only in Russia, Poland, and Switzerland. A small tape-worm, _Tænia nana_, is observed nowhere except in Abyssinia; the _Anchylostoma_ is known at present only in the south of Europe and the north of Africa; the _Filaria_ of Medina, in the west and the east of Africa; the _Bilharzia_, that terrible worm, has only been found in Egypt.

There are also parasitic insects dreaded by man, as the _Chigoe_ (_Pulex penetrans_) which, happily, is only known in certain countries. Some, however, have become cosmopolitan, since man has introduced them wherever he has established himself.

The mammalia which live on vegetable diet have Tænia without any crown of hooks, and man, according to his teeth, ought only to nourish the _Tænia mediocanellata_. We find in a work on the Algerian Tænia, by Dr. Cauvet, that it is the _Tænia inermis_, that is to say, without hooks, which is the species common in Algeria. Among fourteen tæniæ which he had occasion to examine, there was not a single _Tænia solium_. I have said long since, that this species ought to be less widely spread than the tænia without hooks. The _Tænia solium_ comes 106 from the cysticercus of the pig, the other from that of the ox; and Dr. Cauvet has ascertained that the latter, in the state of cysticercus, has already lost its crown.

We find extinct fossil genera and species in all the classes of the organic world. Is it the same with worms and animals of other classes which are only known in the condition of parasites? Had the Ichthyosauri and the Plesiosauri worms in their spiral coecum like plagiostomous fishes, which resemble them so much in the digestive tube? We do not doubt this, and we should have been glad to give some demonstration of it. For this purpose, we have made a collection of the coprolites of these animals, but we have not yet succeeded in getting slices thin enough or sufficiently transparent to discover the eggs or the hooks of their cestode worms.

Not long ago, the partisans of spontaneous generation found in the class of worms their principal argument for their old hypothesis, and it was even after the publication of my treatise on intestinal worms that this question, which seemed forgotten, was taken up again by Pouchet. At present, they appear to have given up parasites, which reproduce their kind like other animals, and to have fallen back upon the infusoria, the last intrenchment which remained to the partisans of spontaneous generation, whence Mons. Pasteur has scientifically dislodged them. It is evident to all those who place facts above hypotheses and prejudices, that spontaneous generation, as well as the transformation of species, does not exist, at least, if we only consider the present epoch. We are leaving the domain of science if we take our arms from anterior epochs. We cannot accept anything as a fact, which is not capable of proof.