A small worm, of the size of a slender pin, but much shorter, lives in a manner somewhat analogous to that which we have before described. It is known under the name of _Leptodera_. In order to find it, we have only to search in the woods for the first snail that we meet with, which is distinguished by its orange or black colour: if we prick with a pin the fleshy foot of the mollusc, we shall see torrents of round worms come out, wriggling like microscopic serpents. These worms also leave their retreat, if we cause the foot to contract by touching it with some acid, or if we place the snail in water. The Leptoderæ are especially remarkable for two fringes which float by the side of their tail, which characteristic suggested the name given to them by Professor Schneider. These fringes so easily fall off, that the greater part of those which have become free have none of these appendages. When placed in fresh or decaying animal matter, in water or in damp earth, these worms, agamous when in the foot of the mollusc, rapidly become sexual and perfect. Thus the snail serves them as a _crèche_, and the adult worm has no need of external help when it has grown old. Professor Pagenstecher found at Ostend, on the _Nicothoë_ of the lobster, nematodes which he arranged among the Leptoderæ. This is 155 another instance of a parasite on a parasite. While speaking of these worms, I will allude to a nematode which I observed under very singular circumstances. I had a considerable number of skeletons or, I should rather say, separate bones, exposed to the sun upon a roof to whiten; among these skeletons there were several hyperoodons and other cetacea. All these bones had remained for a certain time in horse-dung in order to hasten the decomposition of the soft parts. They had been in the open air for several weeks, and were slowly bleaching; it had rained nearly every day. Towards the end of the month of August, I examined some of the vertebræ, and found them quite black on the upper part. Below, I discovered a mass of syrupy matter, slightly yellow, like pus that has recently issued from a wound. The sun was shining full upon the bones at this time; looking at them more closely, I saw this pus issuing from the holes which convey nourishment to the substance of the vertebræ; it seemed that the inside of the bones was in full fermentation. Examining it with some attention, I perceived that the whole surface was in motion; an undulatory wriggling covered it as if a ciliated skin had been stretched above the orifices. I took a little of this matter on the point of a scalpel, and observed it with the microscope, and what was my astonishment when I saw the whole mass in motion as if under the influence of a magic wand. When I slightly compressed it afterwards between two slips of glass, there remained nothing before my eyes but nematode worms of very small size wriggling over each other: I found