The Weberian apparatus of Siluri with fully developed swimbladders is described. and comparisons are made with the Cyprini. Special attention is paid to the histology of the soft parts. The reduced encapsulated swimbladders of Clarias, Plecostomus (Loricariidae) and Hoplostemum (Callichthyidae) are described, and the homologies of the parts of their capsules and the mechanisms of their Weberian apparatus considered. It is suggested that reduction of the swimbladder in such forms is in adaptation to life on the bottom or in compensation for the hydrostatic effect of accessory respiratory organs, and that encapsulation is due to the bony parts concerned not being correspondingly reduced.