38 



MORPHOLOGICAL TYPES 



get pushed out to the 

 edge of the jaws, and act 

 as biting teeth. When 

 worn out, they are re- 

 placed by others which 

 are pushed up in their 

 turn to the biting edge 

 (see Fig. 122, p. 263). 



Lateral Line. — In vari- 

 ous places over the body, 

 there are peculiar organs 

 belonging to what is 

 known as the lateral-line 

 system. Essentially, they 

 take the form of canals 

 sunk beneath the skin, 

 and opening to the ex- 

 terior at intervals. In 

 these canals are sense- 

 organs whose probable 

 function it is to appre- 

 ciate low-frequency vibra- 

 tions in the water. One 

 of these canals runs along 

 the side of the body from 

 the tail to the head, and 

 is the true " lateral-line 

 canal." At about the 

 level of the first gill-slit, 

 it gives off a transverse 

 occipital canal which runs 

 over the top of the head 

 and meets its fellow from 

 the opposite side. It con- 

 tinues forwards over the 

 spiracle as a short tem- 

 poral or postorbital canal, 

 and divides into two. One 



