XV.—Development and Probable Evolution of the Suctorial Disc in the Tadpoles of Rana afghana Günther

Extract

The remarkable and close parallelism exhibited by the suctorial disc of the fish Garra and of certain tadpoles belonging to frogs of the section Rana formosa of Rana was pointed out by the late Dr Annandale and myself in 1922. Evidence was then adduced to show that such structures have come into existence through a gradual accumulation of small changes, which probably originated in direct response to the increasing swiftness of the currents as the animals invaded higher and higher levels of torrential streams. The probable evolution and the development of the disc of Garra have been described already (Hora, 1921, pp. 639–643); whereas the peculiar structure of the disc in the tadpoles has hitherto been known only in its fully-formed condition. It seemed probable that, with the discovery of the earlier stages in the development of the disc, the line of evolution of the sucker would prove to be similar to that of the sucking disc of the fishes of the genus Garra (Hora, 1930, p. 241). The observations described hereafter confirm this view.