The molluscs which are the most remarkable from the point of view from which we are now considering them, are the _Entoconchæ_; they live in Enchinoderms, and it was thought for a while that we could see in them an example of the transformation of one class into another. Some years since J. Müller found in a Synapta from the Adriatic, tubes with male and female organs, without any other apparatus, and in these tubes appeared eggs, whence this great physiologist saw molluscs proceed, with a helicoid shell, similar to that of a small natica; he gave them the name of _Entoconcha mirabilis_. Professor Semper has since discovered another species of these, which he has dedicated to the illustrious physiologist of Berlin, and which he found attached to the cloacal sac of the _Holothuria edulis_. The true relation between these molluscs and the holothurians remains 38 to be discovered, and how the entoconchæ become at last simple sexual tubes. At present we must admit that it is the result of a retrogressive development like that of the peltogasters, which, like them, lose all the attributes of their class. They ought, perhaps, to be placed farther on, among parasites. Some years since, some molluscs were observed which have compromised more or less the dignity of their class. Gräffe cites a species of the genus _Cypræa_, which one would certainly not expect to find in this category; it lives among the Viti Islands, in the compartments of the _Milithæa ochracea_. We have referred to it before. Naturalists have given the name of Melithæa to a very beautiful polyp which forms colonies of two or three metres in height. Mons. Steenstrup, with that perspicacity which discerns the most complex phenomena, has also described _Purpuræ_ which live as messmates with the Antipathes and the Madrepores. Quite recently, indeed, Mr. Stimpson has observed in the port of Charleston, a gasteropod mollusc, similar to a Planorbis (_Cochlioelepsis parasitus_) which lives as a messmate in the body of an annelid (_Ocoetes lupina_).