254 ANATOMY OF VERTEBRATES. 



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

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

 jwesent day. It was the faslrion of tail (fig. 127) which prevailed 

 in Fishes throughout the paleozoic 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 vertebraj, and such a jiroportion of the 

 dermoha3mals, as leads to an equal-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 sjiecies presenting it. 

 The embryos and j'oung of Salmonida, of most Malacopteri, and 

 of many Acantlioytcri, go through closely analogous stages to 

 those which were jDcrmanent in extinct fishes ; and the slight 

 upward twist of the coalesced terminal caudals, and the inequality 

 of its up2)er and lower lobes, indicate the fact in the symmetrically- 

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

 fig. 34, and Scomlieroids, fig. 33, the terminal tail-vertebras shrink 

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

 dermohajmal rays are equally developed, 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 heteroccrcal stages at which it is seve- 

 rally arrested in different older extinct species, doubtless in close 

 relation with the power and kind of swinnning required l_iy each. 



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

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

 to bring the inferioi'ly-opening mouth to bear upon the victim. 

 Tlie Sturgeon maintains its body in the obli(iue position while 

 u})turning 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 eftcctive 

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

 angidar, tlic 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. 

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



' See the persistent " trace of the emhvyonal hcteroecrcnl form of the tail " in the 

 Sea-perch {Centropnslis ghjax, Owen), No. 191, p. 51, xuv. 



