During a short stay at Plymouth, in 1889, I was engaged in studying certain points in the anatomy of Cirripedia; finding, however, that a knowledge of the embryology was necessary in order to arrive at a complete understanding of the adult structure, I became wishful to investigate the life-history of some one member of the group. This I had an opportunity of doing at Naples, where I was appointed to occupy the Cambridge University Table at the Zoological Station for a period of six months, subsequently increased to nine. I here succeeded in obtaining a practically complete series of stages of Balanus perforatus , Bruguiere, as well as many stages in other members of the group. Though a number of able observers have occupied themselves with the embryology of Cirripedes, yet, owing to lack of opportunity, and to the difficulty of obtaining complete series of developmental stages, as well' as to the inherent difficulties in the subject, much remained to be done in this line. Willemoes-Suhm alone, with the advantages afforded by his position during the Challenger Expedition, has hitherto obtained a complete series of stages of any one form, but he failed to trace the history of the earlier stages, and in the later, limited himself to the appearance of fresh and spirit specimens, as seen without cutting sections. In fact the method of sections has been little applied to the development of Cirripedes, and not at all to the earlier stages. There is, therefore, little apology needed for an account embracing the results obtained by the employment of some of the more modern methods of embryological study.