IO4 



PISCES 



extinct Pisces. It is generally represented by one or two dorsal 

 fins, one anal (post-anal) fin, and a caudal fin formed of a dorsal 

 (epichordal) and a ventral (hypochordal) lobe. The name diphy- 

 cercal has been given to the primitive symmetrical caudal fin ; and 



it has been shown that the 

 asymmetrical, heterocercal, 

 caudal fin could be derived 

 from it by a bending upwards 

 of the notochordal axis, ac- 

 companied by a reduction of 

 the epichordal and an enlarge- 

 ment of the hypochordal lobe 

 (Fig. 46). A further step in 

 specialisation gives rise to the 

 homocercal type, found in the 

 Teleostei, by the shortening of 

 the axis, the more complete 

 suppression of the dorsal lobe, 

 and the greater development 

 of the ventral lobe (Figs. 63, 

 64). The homocercal caudal 

 fin acquires a secondary out- 

 ward symmetry. As Huxley 

 [226] and Agassiz [2] long ago 

 showed, fish with the more 

 specialised tails pass through 

 the more primitive stages in 

 development : from the di- 

 phycercal to the heterocercal, 

 and thence to the homocercal 

 type (Fig. 65). But in the 

 process the middle or axial 

 lobe, in which runs the noto- 

 chord, becomes relatively re- 

 duced ; so that, whereas in 

 primitive diphycercal and 

 heterocercal tails the noto- 

 chord reaches to or near the 

 tip (Elasmobranchs, Dipnoi, Chondrostei), in the more abbreviate 

 heterocercal and homocercal tails of the Holostei the hypochordal 

 fin projects far beyond it (Figs. 61-66). 



As will appear later, it seems probable that the heterocercal tail 

 has been independently acquired in more than one group (p. 233) ; 

 and that, in some cases, fish whose ancestors possessed asymmetrical 

 caudal fins have reacquired an outward symmetry, making them 

 superficially similar in appearance to the primitive diphycerca 



FIG. 65. 



Successive stages in the development of the 

 homocercal tail of the Flounder, Plewroncctes flesus, 

 Ii., showing the disappearance of the axial lobe, c, 

 and growth of the hypochordal fin, h.f. ac, actino- 

 trichia ; h.a, haemal arch ; hy, hypural cartilage ; I, 

 dermal ray ; n.sp, neural spine ; nt, notochord. 

 (After A. Agassiz.) 



