males by the side of their females; in the bodies of the latter 156 were eggs ready to be laid, and millions of embryos of every age rolling over and struggling among the full-grown worms. Is this a species of worm new to science? Is it a worm which lives in freedom here, and parasitically elsewhere? The first female which presents itself allows us to answer this question. It is not a parasitical worm, at least under this form, because each female contains only one or two eggs. Parasites have so few chances of arriving at their destination, that two young ones would not be sufficient. They must have hundreds or thousands, and then the chances are against them. This worm is evidently a _Rhabditis_, but is it that which lives in the earth, or an allied species? Future observations will perhaps enable us soon to reply to these questions. We do not think that these creatures could have been brought with the bones from the Shetland Isles; they came rather from the horse-dung, and they multiplied beyond measure in the spongy tissue of the bones, where they found good cheer and a convenient lodging. A worm very nearly allied to this exists in abundance in the dung of the cow, to which our regretted colleague, the Abbé E. Coemans, had directed my attention, at the time when he was studying the _Pilobolus cristallinus_. That which decided us to make mention of the nematode of the bones, is the singular history of an ascaris of the frog, whose young ones resemble their parents neither in size, form, or manner of life. There is one generation which can provide for themselves, and is composed of males and females; and another which requires assistance, and only consists of females; unless, indeed, those of the male sex are hidden among the eggs; we refer to the _Ascaris nigro-venosa_, the principal 157 characters of which have been made known by Professor Leuckart. This Ascaris is a true parasite, which, when it arrives at its destination, where it finds lodging and food, leaves the lungs to go and inhabit another organ. There is nothing surprising that certain worms pass from the intestines to the stomach, mount thence to the oesophagus, and sometimes come out of the mouth; but here we have decided changes of abode in the same animal; that which shows, besides, that it is not a simple accident, is that the animal is of a different sex according to the apartment which it occupies; here, it is hermaphrodite, there it is male and female. The Linguatulæ, indeed, migrate from the peritoneum of the rabbit to the nasal fossæ of the dog: but the _Ascaris nigro-venosa_ first lives in the lungs of the frog, then goes to inhabit the rectum of the batrachian, or damp earth. In the lungs it is very small and viviparous, and produces young ones which become stronger than their parents. The generation which live in the lungs are hermaphrodite, the others are dioecious; that is to say, the males and females have hermaphrodites for their parents.