We have brought together in the former chapter the animals which live at the expense of their neighbours, without seeking for anything except shelter. They seize their prey as they pass, are nourished by the blood of their neighbours, but never think of establishing themselves in their organs during any period of their life. They are almost as much carnivora as parasites, and only differ from the former class because they spare the life of their victims. They are unlike ordinary parasites, since they are contented with their food alone; and their appearance from the period of their entrance into the world is that of free animals. Those whose history we are now about to sketch, live in freedom like the preceding during all the time that they are young; like them, they are completely independent during the first period of their life; but when they have arrived at mature age, when the endless cares entailed by their young ones come upon them, they change their costume and accommodate themselves as well as they can to the new lodging which they have chosen. There is often not the least resemblance between these creatures in their youth and their adult state. All these parasites have lived a joyous life
before choosing the host which is to serve them as a cell; but though in many species we see both sexes shut themselves up as in a cloister, some species are to be found in which the female alone seeks for extraneous aid; which is not surprising, since she alone undertakes all the charge of the family, and this would be beyond her strength, and would endanger the life of her offspring, if she did not receive help and protection.
The host resembles in some respects a lying-in hospital, especially when the female alone seeks for herself a resting-place and her food, which is not always the case. We find, in fact, in a considerable number of Lernæans, that the microscopic male passes unperceived upon his female, and when he renounces his bachelor life, she feeds him with her own blood. There cannot be a more faithful husband, since he only plays the part of a spermatophore. We find a still more curious example in this respect, and in which the dignity of the male is not less compromised; we refer to the Bonelliæ which live freely in the sand, and whose males establish themselves parasitically on the sexual organs of the female. She herself lives by her own industry, nourishes her husband, and alone provides for all the requirements of maternity.
In a later part of this work, we shall mention worms which live in freedom in damp earth, and whose direct progeny, entirely composed of females and hermaphrodites, can only exist as parasites. These worms do not resemble their mother but their grandmother, and if their descent had not been traced, they would doubtless have been taken for species entirely distinct from each other. Thus it is not always the whole family which is
modified; the male often preserves all the attributes of his sex and of his youth, while the female changes entirely her appearance and her mode of motion, especially at the approach of the period when the interest of the species prevails over that of the individual.
We can nowhere find more graceful and regular forms during the whole of their early youth than those of many of these parasites; we can never see more ungraceful, we might almost say more comical, attitudes than those of the greater part of these creatures when full grown. One might take them for some misshapen excrescence, or some scrap of wasted flesh on the body of their host. A certain number of insects are found which lead this singular kind of life, but this is more especially the case among the crustaceans, particularly the copepod crustaceans. Among all these we find the most absurd recurrent forms; in fact these animals instead of carrying on their evolution, like the caterpillar which becomes a butterfly, retrograde rather than advance, and acquire an appearance and character which prevent us from recognizing their origin. Many of these are at present known, whose graceful form is so completely changed, that without referring to the study of their embryo state, one could not tell to what class they belong. Nothing remains of their organs except the sexual apparatus and a shapeless skin. These curious parasites live also on the surface of bodies, and sometimes in the cavity of the mouth; but in fishes they are most frequently found in the branchial membranes. They look like natural setons, and it is not impossible that they sometimes fulfil the same functions.
We will first examine some insects, then certain
isopode crustaceans, an order to which the Cloportidæ (wood-lice) belong, many of which require uninterrupted assistance; then we will turn to the Lernæans, which surpass all the rest in their many and bizarre transformations.
