36 



MARKETABLE BRITISH MARINE FISHES 



furnishes important marks for the 



recognition and classification 

 of species. This apparatus 

 consists usually of tubes in 

 the skin opening by rows 

 of pores on to the surface, 

 and containing in their 

 walls minute structures 

 connected with the ends of 

 nerves. It is probable that 

 the function of this appar- 

 atus is to enable the fish 

 to perceive movements of 

 the water such as those 

 caused by the passage of 

 another fish, but very little 

 is known with any certainty 

 on the subject. Whatever 

 the function the general 

 arrangement of the tubes 

 is very constant : there is 

 usually one down each side 

 of the body from the gills 

 to the tail, and the course 

 of this being visible ex- 

 ternally forms what is 

 called the lateral line (L.L., 

 Fig. I.). In front this 

 tube divides into several 

 branches on the head, but 

 these are not usually con- 

 spicuous on the surface. 



The class of fishes con- 

 sists of a number of very 

 distinct orders which differ 

 very widely from one an- 

 other, and in some of which 

 many of the characters 

 above described are want- 

 ing. These characters in 

 fact are only found all 



