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 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 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 Cœnurus 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 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.