The ingestive cycle of Onchidoris is described based upon an anatomical investigation of the buccal apparatus together with observations of feeding activity and spontaneous and induced movements of the buccal apparatus. Feeding is accomplished by a complex series of movements of 25 buccal muscles operating the odontophore and the buccal pump. Protraction of the buccal apparatus is largely passive and is dependent upon increased blood pressure within the head whereby blood is isolated in a number of distinct haemocoels surrounding the buccal apparatus. The odontophore is everted through the mouth by the contraction of ventral and lateral odontophoral protractor muscles and the radular teeth are erected by a deformation of the odontophoral cartilage. The effective stroke of the radula results in the piercing of the barnacle mantle and the opercular plates are discarded by ciliary currents around the mouth of the dorid. Retraction of the buccal apparatus follows due to the contraction of buccal retractor and retractor of the buccal lip muscles and this is accompanied by the inhalant phase of the buccal pump. As a result of these last activities, all the soft parts of the barnacle, including the cirri, are taken into the buccal cavity. It is suggested that Onchidoris represents a high degree of specialization in its feeding behaviour because, although many of the buccal muscles are homologous with those described in other opisthobranchs, their methods of operation, particularly in conjunction with a piercing, grasping radula and a buccal pump that can operate during both ingestion and swallowing, are unique. The buccal apparatus of Onchidoris is compared with that of a number of other opisthobranchs including Philine, Cylichna and Archidoris.