196 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



obtaining access to them through the pores. In many teleosts (fig. 

 203) the pores pass through notches or openings in the scales, while on 

 the head the canals themselves frequently run through some of the 

 cranial bones. 



Of considerable morphological importance, especially in connexion with the 

 morphology of the ear, are the facts that the sensory areas multiply by elonga- 

 tion, followed by division, and that the pores themselves increase in the same 

 way (fig. 202); the pore elongates and then its margins meet in the middle, 

 thus producing two pores. There has been much discussion as to the develop- 

 ment of the lateralis nerves, especially that of the trunk, some thinking that it 



Fig. 204. — Head of pollack, showing lateral-line canals and nerves of the lateralis 

 system, after Cole. Lateralis nerves black, canals and brain dotted, b, buccalis ramus 

 of VII nerve; dl, dorsal ramus of lateralis of X nerve; A, hyomandibularis nerve; hm, 

 hyomandibular line of organs; to, infraorbital line; /, lateral-line canal; n, nares; 0, 

 olfactory lobe; op, operculum; os, ophthalmicus superficialis nerve; soc, commissure 

 connecting lines of the two sides; so, supraorbital line of organs; st, supratemporal part 

 of lateral line; vl, ventral ramus of lateralis of X nerve; x, visceralis part of X nerve. 



increases by additions from the ectoderm of the skin. It appears more probable 

 that all of its material is derived from the nerve and that there are no additions 

 from other sources. 



The distribution of these organs and their canals varies consider- 

 ably. The most constant Hnes are the following (fig. 204) : A su- 

 praorbital line running forward from the region of the ear, above the 

 eye, to the tip of the snout and innervated by the superficial ophthal- 

 mic branch of the seventh nerve; an infraorbital line running in the 

 same way beneath the eye and supplied by the buccalis nerve; a 

 hyomandibular line extending along the lower jaw (and the oper- 

 culum when present), and innervated by the mandibularis externus; 



