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er 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.

Any one who wishes to make observations on distomes in the state of 198 cercariae has only to examine some fresh-water molluscs, either the Limneae 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 cercariae, which are scattered among as many different inferior animals. Birds and fishes become infested by them in consequence of eating these animals.

Another distome was also found by Bilharz in the intestines of a young Egyptian boy.

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 cercariae during the time that they inhabit the water. Larvae and their parasites ought to be carefully studied in the localities where bats abound.

The name of Monostoma has been given to some of these trematodes which have no abdominal sucker.

The embryo, having long ciliae in front, and in the interior a sporocyst already full of young cercariae, 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 fossae 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 millim?tres in length , and are found in the titmouse, the siskin, the sparrow, the canary, and some other birds.

The worms which naturalists call Cesto?ds, or Cestodes , 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 taeniae, both incompletely developed, and 206 occupying the digestive tube. One has a rostellum without hooks, like the taeniae 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 . 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.

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.

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 taenia 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.

"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 , 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 taenia, or more properly the scolex. As long as the mother is there, she engenders and produces cucumerinae, 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 taenia 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 taenia, 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.

The cat entertains another kind of taenia, 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.

Let us consider the solitary worm of man , it will enable us to understand all the others. Known by the name of taenia, 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 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.

The cysticercus of the pig, when introduced into man, becomes a taenia 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 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. Taenia 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 taenia. Ruppell and many others have noticed this fact. Mons. K?chenmeister 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 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.

These vesicles in pork come from the eggs which the taenia has scattered in its passage, and if the pig comes by chance in contact with the fecal matter of a person infested by one of these worms, it is soon infested and becomes what is called measled; in this fecal matter there are either free eggs which have been evacuated by the worm, or else fragments, known long since under the name of cucumerinae, which are full of eggs.

These fragments of taenia, which I have proposed to name proglottides, and which are nothing else than the worm in all its sexual maturity, are still living and wriggling at the moment of their evacuation, or else they are dead and often completely dried; but in either case, they are full of eggs. Each egg is surrounded by membranes and shells, which effectually protect it against all dangerous contact.

A fragment of the mature taenia, thus filled with eggs, when introduced into the stomach of the pig, is rapidly digested, and the eggs are set at liberty. These lose their shells by the action of the gastric 218 juice, and there issues an embryo singularly armed. As we have before said, it carries in front two stylets in the axis of the body, and on the right and left sides two other stylets curved at the end, which act like fins. These embryos bore into the tissues as the mole burrows into the soil. The middle stylets are pushed forward like the snout of the insectivore, and the two lateral stylets act like the limbs, taking hold of the tissues and forcing the head forwards. In this manner the embryos perforate the walls of the digestive tube.

Scharlau, at Stettin, found taeniae in seven children who had been fed, on account of anaemia, with raw meat. The taeniae were those of this species. We have ourselves found them in children to whom the use of raw meat had been prescribed.

We do not think it necessary to speak here of a third species of taenia , which also lives at our expense, but which has been 220 hitherto found only in Egypt.

This microscopic embryo is armed with six hooks, like embryos of all the cestodes; it employs them with much dexterity to pierce the walls of the organs, and to hollow out a space for itself in the substance of the tissues. Shut up in its hiding-place, membranes form around for its protection; its six hooks, having become useless, wither; other 221 hooks in the form of a crown appear by the side of four rounded projections, the future suckers; and, sheathed in a large vesicle full of a limpid fluid, it waits patiently for the moment when it will find a place in the stomach of a dog. If good fortune awaits it, it will wake up, some fine day, in the stomach of the animal which has eaten the rabbit, its former home, and a new life will commence for it. The organs in which it was imprisoned are digested, it gets rid of all its swaddling-clothes, unrolls itself, separates from the vesicle which has protected it hitherto, and penetrates into the intestine; there, immersed in the food of its host, it grows with extreme rapidity, and assumes the form of a ribbon or tape. The ends of this tape are successively matured, detach themselves, and become the complete worms, full of eggs, which are evacuated with the feces; scarcely have they made their appearance in the open air before they burst and scatter their eggs.

It was with these cysticerci that I made experiments on four dogs, 222 which I took with me to Paris, in order to convince those who could not believe in the migration of parasites. It was this species that I gave also to the dogs which served as a demonstration at Paris at the course of lectures given by Mons. Lacaze Duthiers.

A fortnight later, that is to say, about the thirty-second day, the coenurus is as large as a small nut, and one can see with the naked eye some small nebulous corpuscles, separate from each other, of the same form and size; these are the buds or scolices which have risen up, but which, as yet, have neither hooks nor suckers.

Eggs of the same taenia have been given to sheep at Copenhagen and at Giessen, and Messrs. Eschricht and R. Leuckart have obtained the same result as we had at Louvain. On the fifteenth or sixteenth day the first symptoms of "gid" declared themselves. At about the thirty-eighth day the crown of hooks appeared, the suckers were formed, and the whole head of the scolex was sketched out. All these heads can leave or enter the sheath at the will of the animal. It is truly a polycephalous animal when the scolices are expanded. This worm continues to grow for a long time in the cranial cavity, and produces by its presence the gravest results. The sheep necessarily dies at last, unless we remove the parasite by means of the trepan.

The coenurus, at this point of development, swallowed by a dog, undergoes great changes in a few hours. The proscolex, or large vesicle, withers; the different scolices unsheath their cephalic extremity, become free, penetrate into the intestine with the food, and attach themselves to its walls, so as to form as many colonies of taenia as there are distinct heads. A dog which has swallowed a single coenurus may therefore contain a considerable number of taeniae.

To arrest this disease, only one thing is necessary, to destroy by fire the head of every sheep attacked by the "gid." The rest of the animal may be eaten without danger.

Leuckart has made some very interesting experiments on the echinococci. In Fig. 57 is shown a taenia which proceeds from an echinococcus.

A Russian naturalist, Dr. Koch, thoroughly studied this interesting worm and its evolution. He says that this cestode is rare at Moscow, while at St. Petersburg, Riga, or Dorpat it is common. If this be really the case, it must doubtless be attributed to the fact that in one place the inhabitants drink spring water, and in the other water from the river.

A very curious circumstance is the actual rarity of the Bothriocephalus among the inhabitants of the shores of the Lake of Geneva, though formerly it was very common there. This diminution, if we may not call it disappearance, is due to the change which has been made in the construction of water-closets, all of which formerly emptied themselves into the lake, so that the embryos were hatched in the water, and persons were infested by them through drinking it. At present the refuse of the towns is carefully collected for the purpose of manuring the land. This is the result of the advice of Mons. de Candolle, half a century ago; for this naturalist clearly understood how great was the loss to agriculture from the neglect of this fertilizing agent.

The itinerary of this tape-worm is simple. It passes from man to the water under the form of an egg, or of a proglottis; and from the water to man in the shape of a ciliated embryo. In this manner it is introduced with the water that is drunk. The Bothriocephalus, like other cestodes, is free at the commencement and the end of its life: 229 at the beginning, in order to penetrate into its host; at the end, to scatter its eggs.

Almost all birds nourish large and beautiful taeniae, but they must be studied immediately after the death of their host. They often change their form entirely at the end of a few hours.

Woodcocks and snipes always have their intestines stuffed full of taeniae and the eggs of these worms. Every bird contains them by thousands. Fortunately we cannot be infested with the taenia of the snipe and the woodcock.

There are other worms which migrate, and even some articulate animals; but their modifications of form are much fewer than in the preceding, and their changes are generally restricted to simple metamorphoses. We will place at the head of this chapter the Linguatulae, which have so perplexed naturalists.

We have shown, from the embryos, in 1848, that the Linguatulae, instead of being worms, are articulate animals, more allied to the lerneans or acaridae than to the helmintha. These observations, though received at first with much hesitation, were fully confirmed afterwards, especially by the learned researches of Leuckart. The linguatulae have a very long body, sometimes rounded, in other cases compressed, with a mouth surrounded by four strong hooks, regularly disposed in a semicircle. They have often been found in the lungs of serpents, in certain birds, and in many mammals. A linguatula was also seen by Bilharz at Cairo, in the liver of a negro, and they have been observed in the hospitals of Dresden and Vienna.

The nematode worms are long and rounded, like the ordinary ascarides of infants, which take up their abode in all the organs of animals of the various classes of the animal kingdom. About a thousand varieties are known, varying in length from a few millim?tres to forty or 233 fifty centim?tres.

They are not all parasites, as has been thought, since some are found in the sea, and others in damp earth, in putrid matter, and even on plants and their seeds. The migrations of nematodes are subjects of great interest. Their changes of form are usually not very considerable; but the modifications in their sexual apparatus, whether in the same individual, or in the succeeding generations, are very curious.

When we consider the numerous encysted and agamous nematodes, which are found in the different orders of mammalia, birds, reptiles, batrachians, and fishes, there is little doubt that all these beings are only migratory parasites, which pass together with their hosts into the animal to which they are destined. They are found, like ascarides, in animals of all classes. Some are to be met with in all the organs--the brain, the eye, the muscles, the heart, the lungs, the tracheal artery, the frontal sinus, the digestive tube, the skin, and even in the blood. Sometimes the two sexes live under the same conditions; sometimes the male is dependent on its female, or else one generation is parasitical, and the next is independent. There is a great diversity with respect to development. Some nematodes, like trichinae, are developed so rapidly, that the embryos are already perfect in the egg before it has quitted its mother. Others, like the ascarides lumbricoides, lay eggs, in which the embryos do not appear till several weeks or many months after they have been laid. Between these two extremes we find all the intermediate degrees.

These worms are usually very tenacious of life; many of them can, it is said, be dried for weeks, months, or years together, and return to life as soon as their organs are moistened. Their eggs resist even the action of alcohol and the most active chemical agents, and eggs that had been prepared for the microscope, and had served for many years 235 the purposes of study, have been known to produce young ones as if they had been just laid.

Arrangements which would not have been suspected beforehand, are every day revealed, with respect to the conservation of species. We have recently learned from the works of Messrs. Malmgren and Ehlers, and later still, from those of Clapar?de, that in the same species we may find different males, producing different offspring. Messrs. Malmgren and Ehlers have opened this question by their persevering researches, 236 and Mons. Clapar?de expected to invalidate the results obtained by them by establishing himself at Naples, in order to devote himself to a new series of investigations. Contrary to his expectations, he arrived at the same conclusions, and announced that a nereid possesses, in one and the same species, two kinds of males and two sorts of females, and that these males differ from each other, not only in their manner of life but in their age, in the mode of formation of the spermatozo?ds as well as in the form; that the females differ no less from each other than the males, and that each form is intended to provide, in its own manner, for the dissemination of the eggs.

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