In the Memoir published in my Zoological Researches, p. 1, with its sequel, p. 63, having first made known the fact of the Brachyura of the Decapoda (Crabs) passing through the intermediate form of Zoea ; I have now to announce that they undergo another metamorphose, no less singular and unlooked for, in which they assume the form of the genus designated by the name of Megalopa by Dr. Leach, from the disproportionate size of the eyes. This second stage we may therefore consider analogous to that of pupa in the class Insecta . By the former memoir it appears that the young of Cancer Pagurus , the common market Crab, first presents itself as a Zoe, and that a full-grown Zoe was observed passing into some other more perfect form, which at that time was considered to be that of some species of Crab: the discovery now first detailed, however, shows that it must have been only passing into that of a Megalope .