28 NERVE COMPONENTS OF BONY FISHES. 



2. — The Lines of tJie Head. 



The lateral line canal system of the head, like that of 

 the trunk, shows evidence of reduction or arrested devel- 

 opment. The primary lines of the head are present in 

 very nearly the typical form, though in part they are re- 

 presented by lines of naked organs like those of the trunk 

 line. The canals and their organs bear, so far as ascer- 

 tained, the typical relations to the bones, as described by 

 Allis for Amia ('89) and Cole for Gadus ('98a) and pores 

 are present between each pair of organs, the only ex- 

 ception being the space between the first organ of the 

 trunk canal and the organ of the occipital canal, between 

 which there is no pore. The pores are always short and 

 simple, never showing the dendritic arrangement found 

 in some other fishes. The lateral line organs of the head, 

 whether contained in canals or not, are always supplied 

 by the acustico-lateral system of nerves, and these nerves 

 never supply any other organs. 



This is a point which can be determined with certainty 

 in nearly every case by reason of the great size of the 

 lateral line fibres, as compared with those of any other 

 sensory system. These fibres, when bound up in a com- 

 mon sheath with others, usually segregate themselves, so 

 that they occupy a definite portion of the cross-section of 

 the nerve, and this renders their separation still more 

 easy. The most serious difficulties were encountered in 

 distinguishing certain naked organs of the lateral line 

 system, which correspond probably with the pit-lines of 

 Amia, from terminal buds. These organs are sometimes 

 innervated by fibres which are intermediate in character 

 between those of the lateralis and communis systems, 

 being very densely myelinated and hence staining very 



