CHAPTER IX.

PARASITES THAT UNDERGO TRANSMIGRATIONS

AND METAMORPHOSES.

A CERTAIN number of parasites establish themselves at first in an animal which serves as a crèche, then in a second which serves as a lying-in hospital. This passage from one animal to another is described under the name of transmigration. In general, the entire crèche with its nurslings passes into the lying-in asylum. The crèche is always represented by an animal which feeds on vegetable diet, which is destined for one which is carnivorous: the lying-in asylum is represented by the latter. The mouse is the crèche which will pass with all its clients into the cat which eats it.

If we were treating of plants, we should say that in the first host they are developed, and in the second they blossom. The plant, like the animal, is agamous as long as the flower and the sexual organs have not made their appearance.

The animal which migrates usually undergoes a complete change in passing from one abode to another; it is agamous in the first instance, that is to say, without sex, swathed and covered with a padded cap like a nursling; in its last stage it is, on the contrary, endued with all its sexual attributes.

In the crèche the parasite is on its passage from one station to another, and that which arrives at the lyingin asylum has reached the end of its journey and is at home. We have proposed to give it the name of Nostosite, as distinguished from that which only inhabits its host for a time. We may also remark that the same animal may give lodging to these two kinds of parasites. It is thus that the rabbit harbours in its peritoneum passengers which are only at home in the dog; and, independently of these passengers (these strangers may we say ?), it lodges in its intestines a sexual tænoid worm. The first is a Xenosite, the second a Nostosite. The mouse, in the same manner, gives lodging to passengers under the name of Cysticerci, which are destined to the cat in order to become Tæniæ.

We might call the rabbit or the mouse which harbours worms in transitu, the stage coach; more especially as from time to time there are some which miss it, and are consequently lost in their peregrinations.

This stage-coach is the intermediate host, the Zwischenwirth of German helminthologists, which is always an animal with a vegetable diet; the final host is generally a carnivore it is by means of the vegetable feeder, the grazing or herbivorous animal, that the stranger parasite introduces itself.

The result of this is, that the carnivore receives into its house, every time that it devours its prey, all the parasitical inmates of the latter, and the walls of its digestive canal form the soil in which are implanted all the worms which can take root there. The tissues of the prey are triturated and digested, but the worms which it encloses escape the action of the gastric juice,

and are set at liberty in the, stomach. The stomach of of the carnivorous animal is a sieve through which thousands of parasites are often introduced at each repast, and fishes lodge many which often pass from one stomach to another. Their whole life is spent in these migrations; they are travellers who have their abode in railway carriages, and never take their departure at the stations.

Each stomach is, in fact, a station, very frequently quite filled with merchandise, which disappears with the station itself by the next train. Happy are those who find themselves in a carriage safely on the rails towards its destination. Many are called but few chosen. How many journeys some of these travellers have to take before they find their host!

It is often very interesting to open a fish which has made a good meal; its stomach and intestines contain, first of all, the usual worms; the half-digested prey, in its turn, encloses some; and it is not rare to find besides them the parasites of those which were swallowed together with their host.

The animal is usually attacked in its youth by the parasites which it harbours all its life. In order to know the inhabitants of some fishes, we must examine them shortly after they are hatched.

In the crèche the parasite occupies an organ which is closed, and without communication with the outer world; it inhabits the garret of its first host; in its last host, which represents the maternity asylum, it dwells, on the contrary, in the largest apartments, and never ceases to be in direct communication with the exterior. Thus, in the first animal, it is often completely immovable and under a form which we have named scolex; in the

latter it moves freely, and has, in addition to sexual organs, those which are proper to this condition which we have called Proglottis. Thus these parasites undergo metamorphoses.

For a long time, metamorphoses seemed to be the attributes of frogs and insects exclusively. In the class of worms, in which they are complicated with the change of hosts, they much surpass in reality the most brilliant and extravagant fictions of the poets. The phenomena of these transmigrations were completely unknown before our researches were made. If some naturalists, like Abildgaard or Pallas, suspected their existence, it was rather by accident, and the experiments to which they devoted themselves were all unfavourable to their suppositions.

The knowledge of these transmigrations has at the same time dispersed the latest illusions of the partisans of spontaneous generation; it was the more difficult to explain the presence of worms in enclosed organs, since these worms were always without sex. By the same means, we have ascertained the true prophylactic treatment, and thus discountenanced the numerous anthelminthic remedies which had often caused more serious accidents than the parasites themselves.

When it was considered that parasites were the result of an especial degeneration of some of the intestinal papillæ, the physician would at once consider that there was some morbid condition, and we can understand that all his efforts would be employed against the enemy which had arisen. Now it is known that every healthy animal living in freedom contains parasites almost as invariably as the organs which support its

life; and it is not a matter of doubt to us that parasites often play their allotted part in the economy; their absence as well as their presence may be the cause of inconvenience. We should not even be astonished if the administration of certain worms internally should be prescribed as a remedy. Have we not known the time when all maladies were supposed to yield to the action of leeches, and do we not see the good effects of their application? There are many kinds of parasites, and their therapeutic effect may, perhaps, in future, form an interesting subject of study.

To speak at the present time of a verminous temperament would be scientific heresy, an anachronism; this shows the progress that we have made of late years. Valenciennes was permitted to employ this language at the Academy of Sciences in Paris not twenty years ago, and Lamarck wrote thus in his standard work on invertebrate animals, in the beginning of this century: "It is very certain that there exist in a great many animals, and even in man, intestinal worms; some of which are formed there, others are born and all live there, multiplying more or less, without any of these worms showing themselves externally, or being able to live elsewhere.

"During so many centuries that observations have been made, well-ascertained species of intestinal worms have been found nowhere else than in the bodies of animals. We are now authorized to believe that there are innate worms, or such as are produced by spontaneous generation, and that these are modified from time to time; this is at present the opinion of the most enlightened observers.”