Subsequently, a number of cartilaginous pieces are developed in connexion with the dorsal 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 _basi-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, typically 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-dorsals; and the other, _supra-inter-dorsals_, alternating with the former, lie over the inter-dorsals, the two series forming the keystones of the dorsal arches, and thus completing the roof of the neural canal. On the ventral side of the notochord this arrangement 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 downwards into ventral prolongations, they unite in pairs beneath the caudal artery and vein, and so form a series of inverted arches {195}(_haemal arches_) enclosing a haemal canal (Fig. 112, B). The apex of each arch is prolonged into a median process or _haemal spine_. Although not recognisable in the Dog-Fish, paired _inter-ventral_ cartilages, corresponding with the inter-dorsals above, are present in some Elasmobranchs and alternate with the basi-ventrals. In the caudal region of others (_e.g._ Skates and Rays) ventral counterparts of the supra-interdorsals are present, and are termed _infra-ventral_ cartilages. Much in the same way that their dorsal equivalents enclose a neural canal, so the ventral arcualia partially surround the viscera-containing coelom in the trunk; and in the tail, but more completely, the vestigial coelom of that region or the haemal canal.