254 ANATOMY OF VERTEBRATES. 



developement is termed the ' heterocercal ' one. It is shown by 

 the Sturgeon, fig. 29, and by the Chimaeras and Sharks of the 

 present day. It was the fashion of tail (fig. 127) which prevailed 

 in Fishes throughout the palaeozoic and triassic periods. 



In some oolitic fishes first is observed such a lengthening of 

 the dermoneurals of the tail, with such a shortening and running 

 together of the terminal vertebra?, and such a proportion of the 

 dermohtemals, as leads to an eqnal-lobed caudal fin, which has 

 been termed ' homocercal ; ' but as it is only symmetrical in 

 contour, and remains more or less unsymmetrical in its frame- 

 work, I term it ' homocercoid.' The ganoid fishes of the mesozoic 

 periods manifest several interesting gradations of this transitional 

 state from the hetero- to the true homo-cereal form, each step 

 being a permanent character of the extinct species presenting it. 

 The embryos and young of Salmonidce, of most Malacopteri, and 

 of many Acanthopteri, go through closely analogous stages to 

 those which were permanent in extinct fishes ; and the slight 

 upward twist of the coalesced terminal caudals, and the inequality 

 of its upper and lower lobes, indicate the fact in the symmetrically- 

 shaped ' homocercoid ' tail-fin of the adults. 1 In the Anacanthini, 

 fig. 34, and Scomberoids, fig. 33, the terminal tail-vertebrae shrink 

 and coalesce in the line of the trunk's axis ; the dermoneural and 

 dermohamial rays are equally develojjed, and a truly symmetrical 

 or ' homocercal ' caudal fin is the result ; and this is the latest 

 and greatest modification of the organ. The majority of existing 

 species of bony fishes indicate, in the course of their acquisition of 

 the symmetrical tail-fin, the heterocercal stages at which it is seve- 

 rally arrested in different older extinct species, doubtless in close 

 relation with the power and kind of swimming required by each. 



The heterocercal tail helps the fish to vary its onward course. 

 The Shark wheels about in pursuit of prey, and rotates the trunk, 

 to bring the inferiorly-opening mouth to bear upon the victim. 

 The Sturgeon maintains its body in the oblique position while 

 upturning the muddy bottom of the strongly-running stream, and 

 avoids, by deftly bending to right or left, the drift bodies that are 

 hurried down the river. The homocercal tail is a more effective 

 form for a straight forward rush. When it is truncate and tri- 

 angular, the apex being the centre of motion, the centre of force 

 is three-fourths the distance of its base from the axis of oscillation, 

 and the muscles of the tail act at a corresponding disadvantage. 

 When the tail is forked, as in fig. 33, the area is in the inverse 



1 See the persistent " trace of the embryonal heterocercal form of the tail " in the 

 Sea-perch (Centropristis gigas, Owen), No. 191, p. 51, xliv. 



