SENSORY ORGANS. 



181 



(fig. 180); the pore elongates and then its margins meet in the middle, thus 

 producing two pores. There has been much discussion as to the development of 

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



FIG. 181.- Stereogram of lateral line organs of a fish, c, lateral line canal; In, lateralis 

 nerve; p, pores connecting with the exterior; s, scales in skin; so, sense organs of lateral line. 



sac 



FIG. 182. 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; h, hyomandibularis nerve; hm, hyomandib- 

 ular line of organs; io, infraorbital line; /, lateral line canal; n, nares; o, olfactory lobe; op, 

 operculum; os. ophthalmicus superficialis nerve; soc, commissure connecting lines of the 

 two sides; so, supraorbital line of organs; st, supra temporal part of lateral line; vl, 

 ventral ramus of lateralis of X nerve; x, visceralls part of X nerve. 



The distribution of these organs and their canals varies considerably. 

 The most constant lines are the following (fig. 182): A supraorbital 

 line running forward from the region of the ear, above the eye, to the 



