We have thus a mother, a simple female or hermaphrodite, very small, which produces, not eggs but young ones fully formed; and instead of living, like the mother, in the lungs, and breathing there with greater or less facility, they go and lodge in the rectum, and become, not like their mother, viviparous and hermaphrodite, but oviparous and of separate sexes. They produce in their turn a race of giants, and instead of following the example of their father or their mother, they all go and lodge in the lungs like their grandmother. If the hermaphrodite _Ascaris nigro-venosa_ alternately produces 158 individuals of separate sexes, that is to say, if the monoecii produce dioecii, and the dioecii again monoecii, one cannot help comparing this phenomenon to digenetic generation. This is one of the striking discoveries made at the laboratory of Giessen, under the direction of Rud. Leuckart. Since then, Professor Schneider, the successor of Leuckart at the University of Giessen, has also studied these worms. Professor Leuckart wrote thus to me a few days after this discovery: "The _Ascaris nigro-venosa_ presents this peculiar phenomenon, that, under the parasitical form, it produces fertile eggs without the presence of males. The embryos which proceed from the eggs become sexual worms at the end of twenty-four hours after they have left the body. This fact was first observed by M. Mecznikow, while he was working in my laboratory, and taking part in my researches. The experiment which produced this result was suggested and directed by myself, in order to continue my work on the development of the Nematodes." We do not know if this is the place to speak of an animal which excited great attention some years ago, and which was thought to prove the transformation of animals into each other. It is a parasite which, under the form of a gasteropod, lives under peculiar conditions. It is known by the name of _Entoconcha_. Discovered by J. Müller in an echinoderm of the genus Synapta, its complete development has been vainly sought to be discovered since that time. It is evidently a gasteropod mollusc, allied to the Natices, and lives in the interior of the body of a Synapta, but we do not yet know all the phases of its 159 evolution. It was at first thought that we had before us an echinoderm in the act of transformation. I wrote to J. Müller immediately after the discovery which he hastened to announce to me, to state that in my opinion, this was only a new instance of parasiticism; parasites are, however, so rare in this class of animals, and their mode of life is so exceptional, that one ought not to be surprised that this fact did not receive at first its true interpretation.