1 64 



A HISTORY OF FISHES 



arch protecting the spinal cord, and the lower or haemal arch 

 performing a like service for an artery and a vein (Fig. 66a). 

 The lower arches are of two kinds, those in the tail region 

 {i.e. of the caudal vertebrae) being complete and meeting 

 below to form a tunnel, whilst those in the trunk region {i.e. of 

 the abdominal vertebrae) project sideways as short processes 

 to which are attached slender ribs, running outwards in the 

 walls of the body and ending in the partitions between the 

 segments of the body muscles. In some of the more primitive 

 Bony Fishes, such as the Sturgeons {Acipenseridae) and Lung- 

 fishes {Dipneusti) , the vertebrae are still composed largely of 

 cartilage, and portions of the notochord persist between the 

 centra, as in the Dog-fish. There are, however, two pairs of 



neural arch. 



"" tran^t/trse proce^i 



n..a.rch 



aenvaC sptne 



B.2. 



Fig. 66. — VERTEBRAE. 



A. Cross - section through one of the vertebrae of Comb-toothed Shark 

 {Heptranchias perl6),y,\ ', b.i. Lateral view of an abdominal vertebra of Cod 

 (Gadus callarias), X J ; B.2. Caudal vertebra of same, X |. 



additional elements not found in the Selachians : the supra- 

 neurals, united to form neural spines, and infra-haemals, united 

 in the tail region to form haemal spines, but in the trunk region 

 taking the form of what are known as pleural ribs. In most of 

 the higher Bony Fishes the vertebrae are more or less completely 

 ossified, but have the same essential form throughout the class 

 (Figs. 66b. I, B.2). The Gar Pike {Lepidosteus) is unique in the 

 form of its vertebrae, each centrum being convex in front and 

 concave behind. In the remaining Bony Fishes the centra 

 almost invariably have concave surfaces at both ends, although 

 in the Eels {Apodes) they may be flat or even convex in front. 

 In a number of fishes the articulation between two vertebrae is 

 made more effective by the development of little bony processes 

 on the sides of the neural arches or the centra; these project 



