SCALES 



219 



ficial placoid scales fused together, the presence of denticles on the 

 top of modern ganoid scales can only be explained as a new set 

 developed after the scale has sunk below the surface (see Figs. 188-9). 

 On the other hand, it is, of course, possible that the cosmine is only 

 a peculiarly specialised layer of the independently formed scale, 

 since intermediate stages in its formation have not yet been found. 

 The different varieties of scales described above are of great taxo- 

 nomic importance. 



In the primitive Pisces the organs of the lateral -line system 

 have become distributed over the head and body according to a 

 definite pattern (p. 19), and have sunk in a subdermal tube open- 

 ing on the surface by pores (p. 220 and Fig. 195). The pattern of 



Much enlarged view of a piece of the scale of Lepidosteus ossfiis, L. d, superficial denticles ; 

 (7, ganoine layer ; i, inner bony layers, or isopedine ; t, tubules with branching inner ends ; r.c, 

 vascular canal. 



the lateral line remains very constant, on the whole, throughout the 

 Osteichthyes, though it undergoes secondary modification in detail. 

 But the tube in which the sense-organs have sunk necessarily 

 becomes inclosed in the dermal plates of the head and scales of the 

 body. There is thus established a definite relation between the 

 exoskeleton and the lateral line, which is often of great service in 

 tracing the homologies of the various bones concerned. In all 

 primitive Osteichthyes the lateral-line canal on the body pierces 

 through every scale along its course, passing from its upper surface 

 in front to its lower surface behind (Fig. 194). A sense-organ is 

 situated between each pair of scales, and a tube leads to a pore 

 approximately above the centre of each scale. Secondary lateral 

 lines may be developed dorsally and ventrally essentially similar to 

 the main line, of which they are branches. In front the line passes 

 through the dorsal elements of the shoulder-girdle (post-temporal) 



