The median fins are to be regarded as the more primitive. The fundamental structure of the vertebrate, with its median skeletal axis and its great muscular mass divided into segments along each side of the body, indicates that its primitive method of movement was by waves of lateral flexure, as seen in an Amphioxus, a cyclostome or an eel. The system of median fins consists in the first instance of a continuous fin-fold extending round the posterior end of the body--as persists even in the adult in the existing Dipneusti. A continuous median fin-fold occurs also in various Teleosts (many deep-sea Teleosts, eels, &c.), though the highly specialized features in other respects make it probable that we have here to do with a secondary return to a condition like the primitive one. In the process of segmentation of the originally continuous fin-fold we notice first of all a separation of and an increase in size of that portion of the fin which from its position at the tip of the tail region is in the most advantageous position for producing movements of the body. There is thus formed the _caudal_ fin. In this region there is a greatly increased size of the fin-fold--both dorsally and ventrally. There is further developed a highly characteristic asymmetry. In the original symmetrical or _protocercal_ ( = _diphycercal_) type of tail (as seen in a cyclostome, a Dipnoan and in most fish embryos) the skeletal axis of the body runs straight out to its tip--the tail fold being equally developed above and below the axis. In the highly developed caudal fin of the majority of fishes, however, the fin-fold is developed to a much greater extent on the ventral side, and correlated with this the skeletal axis is turned upwards as in the _heterocercal_ tail of sharks and sturgeons. The highest stage in this evolution of the caudal fin is seen in the Teleostean fishes, where the ventral tail-fold becomes developed to such an extent as to produce a secondarily symmetrical appearance (_homocercal_ tail, fig. 4). [Illustration: From _"Challenger" Reports Zool._, published by H.M. Stationery Office. FIG. 3.--_Chlamydoselachus_. (After Günther.)] The sharks have been referred to as possessing heterocercal tails, but, though this is true of the majority, within the limits of the group all three types of tail-fin occur, from the protocercal tail of the fossil Pleuracanthids and the living _Chlamydoselachus_ to the highly developed, practically homocercal tail of the ancient _Cladoselache_(fig. 2).