TEETH AND SPINES. 37 



a series of interconnecting tubes, which open 

 along definite tracts, not always easily traced, on 

 to the surface. In the sharks the lateral line is 

 represented by a groove protected by overlapping 

 shagreen denticles. In the higher fishes the 

 organ communicates with the exterior through 

 apertures in the scales, apertures often tunnel- 

 shaped in form, and on this account rendering 

 the line more conspicuous. 



CHAPTER IV. 



TEETH AND SPINES. 



IN the preceding chapter it was remarked that 

 there existed an intimate relationship between 

 the bony spine-bearing tubercles or "placoid- 

 scales " of the sharks and the teeth of these fish. 

 Let us now look at this statement a little more 

 closely. 



This relationship is certainly not difficult to 

 follow, though it is as certainly one that would 

 not at first have seemed probable. In certain of 

 the shark tribe, the dog-fish, for instance, we 

 noticed that the skin was covered by innumer- 

 able closely set nodules of bone embedded in the 

 skin, and bearing each a small enamel spine. If 

 a young dog-fish be examined just before hatch- 

 ing, it will be seen that the skin with its closely 

 set spiny scales is continued actually into the 

 mouth and covers the jaws. As growth pro- 

 ceeds, and the lips develop, the original con- 



