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 98 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 99 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.