Abstract The author commences by expressing his obligations to Professor Thomson, who gave him assistance in some parts of his work, and every encouragement in the further prosecution of it. Specimens of Peripatus were collected at the Cape of Good Hope during the stay of H.M.S. ‘ Challenger ’ at Simon’s Bay, with a view to the investigation of the development of the animal. A specimen was dissected and at once seen to be provided with tracheae, and to contain far developed young.