194 



FISHES 



CHAP. 



b.d. i.d. s.d. 



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 lasi-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 

 Dog-Fish fused to form single 

 median cartilages. Of the latter 

 there are two sets one the 

 supra -basi-dorsals, or neural 

 spines, are situated over the basi- 



FIG. iii.-A.sideviewofprecaudaiverte- dorsals ; and the other, supra- 

 brae of Scyliium canicula ; B, similar inter-dorsals, alternating with the 

 view of caudal vertebrae, b.d, Basi- P , . . , . . 



dorsal ; e, centrum ; h, basi-ventrai ; former, lie over the inter-dorsals, 



h.s, haemal spine; i.d, inter-dorsal; the two series forming the key- 

 2), parapophysis ; r, rib ; s.d, supra- 



dorsals. The vertical dotted lines Stones 01 the dorsal arches, and 



indicate the limits of neuromeres and thus Completing the roof of the 

 myotomes. The small circles represent 



the exits of the dorsal and ventral roots neural canal. On the ventral side 



of the notochord this arrange- 



of spinal nerves. (After Ridewood. ) 



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 

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

 dorsals above, which grow out laterally into short processes, 

 the parapophyses 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 



