170 THE STORY OF FISH LIFE. 



regions of the body, changes which show, a 

 gradual advance in structure, as a consequence 

 of more perfect adaptation to their environment. 

 The structure of the pectoral fins, the equivalent 

 of the fore-limbs of higher animals, is much more 

 complex, for instance, than is the case with the 

 similar fins in the older sharks, about which we 

 shall speak in the next chapter. So too with 

 the main axis of the body, which we call in our- 

 selves the backbone or vertebral column. In the 

 sharks this is made up of a series of separate 

 hard bodies or vertebrae, each of them shaped 

 roughly, like a dice-box, when seen in section. 

 In the living animal they are joined one to 

 another by their ends to form a long jointed 

 support the vertebral column. Immediately 

 above this column runs a tube, formed by a 

 series of A-shaped arches, one to each separate 

 vertebra. Through this tube runs the spinal 

 marrow. Besides, the vertebrae also bear pro- 

 cesses for the support of ribs and for the protec- 

 tion of blood-vessels, details of which must be 

 sought for in more technical works. This verte- 

 bral column we call a specialised structure, 

 because in the very young or embryo dog-shark it 

 was preceded by a much simpler structure, in- 

 herited from its ancient and more lowly forbears. 

 The transformation of this into the complex 

 vertebral column then is another piece of evidence 

 of specialisation. This simpler type of vertebral 

 column took the form of a continuous, or as we 

 say, unsegmented, gelatinous rod, called the 

 notochord. Such a notochord always precedes 

 the more complex types of vertebral column or 



