The problem of successfully rearing the larvæ of Echinoidea until they had completed their metamorphosis, has been solved by several naturalists. Agassiz was able, for three weeks, to keep plutei alive which he had fished from the sea, at the end of which time they sank to the bottom and became young echini. Bury was able also to rear a few of the plutei of one of the Neapolitan species through the whole of their larval life, and finally Théel has published an account of how he obtained all stages in the development of Echninocyamus pusillus from the result of artificial fertilisation.