TAIL 101 



ligament generally occur a row of spinal elements. These median 

 spines form the connecting link between the axial skeleton of the 

 vertebral column (see pp. 69 and 105) and the appendicular skeleton 

 of the median fins ; possibly (Goette [167]) they should be reckoned 

 rather with the latter than with the former. 



Assuming that the different types of vertebral arches found in 

 fish have been evolved from such a primitive complex structure, we 

 find that in the course of specialisation the basidorsals and basi- 

 ventrals persist generally as the most important elements, while the 

 others tend to disappear, or to fuse with them. 



Both the dorsal ribs and the pleural ribs may be considered as 



FIG. 61. 



Dissected tail of Polijpterns Uchir, Geoftr. (After Kolliker.) a./, anal fin ; e./, epichordal fin ; 

 fit, dorsal finlet ; h.a, haemal arch ; h.f, hypochordal fin ; n.sp, neural spine ; n.t, slightly up- 

 turned tip of the notochord ; r, emloskeletal radial ; sc, scales. The proximal ends of the 

 dermal rays have been cut off to expose the radials and tip of the notochord. This tail is prob- 

 ably secondarily diphycercal. 



prolongations of the basiventrals ; these basals may remain separate 

 or ' autogenous ' when ossified (in many primitive Teleostei (Figs. 

 336, 358), tail of Amia, etc.), or they may fuse with the centra. 

 Either the ribs or the basiventrals may undergo reduction and 

 disappear. 



In the formation of the haemal canal behind the abdominal 

 region it is the basiventral, not the pleural rib which becomes the 

 haemal arch. The haemal spine is sometimes separate (Amia), but 

 often appears to be merely a prolongation of the arch (most 

 Teleosts, Fig. 63), and the ribs may persist as well (Fig. 336). 



In no fish with a heterocercal or homocercal tail are the haemal 

 spines and radials typically represented in the hypochordal fin (p. 104). 



