194 



FISHES 



and ventral surfaces of the notochord, which, as they form 

 portions of a system of dorsal and ventral arches, are termed 

 " arcualia " (Fig. 111). On the dorsal side there are : (i.) a series 

 of paired lad-dorsal cartilages (neurapophyses or neural arches), 

 the two elements of each pair contributing to form the side walls 

 of the neural canal in which the spinal cord is lodged (Fig. 112, 

 A) ; (ii.) a series of inter-dorsal cartilages (intercalary neural 



arches), regularly alternating with 

 the preceding, and completing 

 the walls of the neural canal by 

 filling up the intervals between 

 the basi -dorsals ; and (iii.) a series 

 of supra-dorsal elements, typi- 

 cally also in pairs, but in the 



d.d. id. 



8.d- 



h. hs. 



s.d. 



i.d.- 



.hd. 



Dog-Fish fused to form single 

 median cartilages. 



-P 

 A 



Fig. 111. — A, side view of precaudal verte- 

 brae of ScyUium canicida ; B, similar 

 view of caudal vertebrae, b.df Basi- 

 dorsal ; c, centrum ; k, basi-ventral ; 

 h.s, haemal spiue ; i.d, inter-dorsal ; 

 2^, parapopliysis ; r, rib ; s.d, supra- 

 dorsals. The vertical dotted lines 

 indicate the limits of neuromeres and 

 myotomes. The small circles represent 

 the exits of the dorsal and ventral roots 

 of spinal nerves. (After Ridewood.) 



Of the latter 

 there are two sets — one the 

 supra -hasi- dorsals, or neural 

 spines, are situated over the basi- 

 dorsals ; and the other, supra- 

 mfer-(fo?-s«/s, alternating with the 

 former, lie over the inter-dorsals, 

 the two series forming the key- 

 stones of the dorsal arches, and 

 thus completing the roof of the 

 neural canal. On the ventral side 

 of the notochord this arrange- 

 ment is substantially repeated by a series of ventral arcualia, 

 which, however, are somewhat differently arranged in the 

 trunk and tail. Thus, in the trunk there are : (i.) a series of 

 hasi-ventral or haemal cartilages, corresponding with the basi- 

 dorsals above, which grow out laterally into short processes, 

 the ^aro^op/iyses or transverse processes, and terminate in 

 (ii.) short, slender cartilages — the costal elements or ribs — which 

 may perhaps be regarded as the ventral equivalents of supra-basi- 

 dorsals. The ribs project outwards into the dorsal wall of the coelom 

 and end in the myocommata separating the myotomes of the body- 

 wall. In the tail the basi-ventrals lose their ribs and, growing down- 

 wards into ventral prolongations, they unite in pairs beneath the 

 caudal artery and vein, and so form a series of inverted arches 



