SCALES. 49 



The scales of tlie lateral line are sometimes larger than 

 the others, sometimes smaller, sometimes modified into scutes, 



Fig. 20.- — Cycloid scale from the lateral line of Labrichtliys laticlavius (magn.) 



sometimes there are no other scales beside them, the rest of the 

 body being naked. The foramina of the lateral line are the out- 

 lets of a muciferous duct which is continued on to the head, 

 running along the infraorbital bones, and sending off a branch 

 into the prseopercular margin and mandible. In many fishes, 

 as in many Scisenoids, Gadoids, and in numerous deep-sea 

 fishes, the ducts of this muciferous system are extraordinarily 

 wide, and generally filled with mucus, which is congealed or 

 contracted in specimens preserved in spirits, but swells again 

 when the specimens are immersed in water. This system is 

 abundantly provided with nerves, and, therefore, has been 

 considered to be the seat of a sense peculiar to fishes, but 

 there cannot be any doubt that its function is the excretion 

 of mucus, although probably mucus is excreted also from the 

 entire surface of the fish. 



The scales, their structure, number and arrangement, are 

 an important character for the determination of fishes ; in most 

 scaly fishes they are arranged in oblique transverse series ; 

 and as the number of scales in the lateral line generally cor- 

 responds to the number of transverse series, it is usual to 

 count the scales in that line. To ascertain the number of 

 longitudinal series of scales, the scales are counted in one of 



E 



