There are some which, like leeches, seek assistance from their 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 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 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 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.