52 



LATERAL LINE 



ings already noted in Fig. 65. A process of this kind 

 is carried to great lengths among the fishes which 

 develop horn-like scales, as Amia, herring, or cod : in the 

 scales of the lateral line region the distal tubules appear 

 at the surface as a cluster of pores, as shown in Fig. 67, 

 or in the detached scale of Fig. 66. 



The organs of the lateral line (of a bony fish) shown 

 in section in Fig. 6'^ are regarded by the writer as of 

 a highly modified character. They appear to have been 

 derived from the conditions of Fig. 62 ; the end organ, 

 S, corresponds with that, S, of the preceding figure ; its 

 size, however, has greatly increased, and the intervening 

 sensory tube has been lost ; its metameral opening at 

 the surface corresponds with that of Fig. 62 ; the nerve 

 supply, N, is now seen to have secured a more perfect 

 relation to the end organs. 



The original significance of the lateral line system as 

 yet remains undetermined. As far as can be judged from 

 its development, it appears intimately, if not genetically 

 related to the sense organs of the head and gill region of 

 the ancestral fish : in response to special aquatic needs, it 

 may thence have extended further and further backward 

 along the median line of the trunk, and in its later differ- 

 entiation acquired its metameral characters. 



A significant feature of its development is its peculiar 

 innervation. Its lateral tract is innervated by a specially 

 evolved root of the vago-glossopharyngeal group, but its 

 head region is supplied by a similar root of the facial 

 nerve (perhaps also by the trigeminus ; cf. Collinge, Ref. 

 p. 248). 



In view of this innervation, the precise function of this en- 

 tire system of end organs becomes especially difficult to de- 

 termine. Feeling, in its broadest sense, has safely been 



