200 



FISHES 



CHAP. 





Inter- ventrals appear to 'BiPlfeeiit. Each neuromere corresponds 

 with a pair of basi-ventrals, of basi-dorsals and of inter-dorsals. 

 The haemal arches and spines are formed partly by the basi- 

 ventrals, but mainly by the ventral union of the successive pairs 

 of ribs. As in the Holocephali, the terminal arcualia of the tail 

 become fused into a straight axial cartilaginous filament, trans- 

 versely divided into segments, which replaces the notochord. 



FlQ. 115. — Side view of tlie precaudal vertebrae of a Sturgeon [Acipenser sturio). a.c, 

 Aortic canal, formed by the median union of ingrowtlis from tlie basi-veutrals and 

 inter-ventrals of opposite Rides ; b.d, basi-dorsals ; b.v, basi-ventral ; i.d, inter- 

 dorsal ; i.v, inter - ventral ; vt, notochord ; n.c, neural canal ; 7i.sjt, neural spine ; 

 nt.s, cuticular sheath of the notochord ; 2^t parapophysis ; r, rib ; s.7t, aperture for 

 the root of a spinal nerve. 



Each segment supports a variable number of dorsal and ventral 

 gable-pieces, or neural and haemal spines. Certain of the verte- 

 bral components, such as the ribs, and the neural and haemal 

 spines, are ensheathed by membrane bone. 



With certain modifications in details the preceding descrip- 

 tion will also apply to the vertebral column of the Chondrostei 

 (Fig. 115). It will be noted, however, that the inter - dorsals 

 are much better developed than in the Dipnoi, although when 



