The bladder of frogs lodges a very beautiful and large trematode which has lately been studied by many naturalists, the _Polystomum integerrimum_. Many observations remain to be made on the different phases of the existence of this parasite. Its organization is known, and it has been seen to lay large and beautiful eggs, but its movements have not been observed before its entrance into the bladder. This Polystomum of the frog--and it is no doubt the same with the species _Polystomum ocellatum_ which inhabits the mouth of the European tortoise (_Emys Europæa_)--lays eggs only in winter, and the eggs of the young ones do not seem to produce more precocious embryos than those of the adult. The embryos are ciliated, unlike those of many of the ectoparasite worms. They much resemble the gyrodactyles, especially by their bristles; and like these, they inhabit the cavity of the mouth before they migrate into another organ. We may even ask if these singular gyrodactyles, so peculiar in many respects, are not the larval forms of trematodes allied to the polystomum. Several important works have lately appeared on the _Polystomum integerrimum_, by Mons. Stiéda in 1870, by Mons. E. Zeller and Mons. Willemoes-Suhm in 1872. The gyrodactyles, which we have just mentioned, are among the most 262 curious worms that have been discovered during late years. They are of small size, and live in the gills of fishes, often in great numbers, and move with considerable agility. They are armed with very variable hooks, which serve to anchor them; and sometimes a digestive canal and organs of sensation are found in them. The _Gyrodactylus elegans_ bears within it a young one which already has hooks, and in this young one, which is not yet born, we see another generation with the same organs, so that three generations are thus enclosed. The daughter is ready at the moment of her birth to give birth to another daughter. According to another mode of interpretation, the mother and daughter are sisters; the elder is found at the periphery, the younger at the centre. These worms are found abundantly in the gills of the cyprinidæ, or white fishes. We have only to scrape gently the surface of the gills with a scalpel, and thus remove a small quantity of a mucous substance, place it on a slide of a microscope, cover it with thin glass, and examine it immediately with the compound microscope. We cannot repeat this three times without finding gyrodactyles.