loo A HISTORY OF FISHES 



nected with the locomotor activities of the fish. In the Salmon 

 and Trout it is surrounded by definite fatty tissue, and is 

 supported at its base by a splint of bone connected with the 

 outermost ray of the pelvic fin. 



The lateral line, a conspicuous feature of most Bony Fishes, 

 will be dealt with in detail in the chapter devoted to sense 

 organs {cf. p. 198), and it will suffice to point out here that it 

 consists of continuous grooves or canals in the head and body 

 containing special sensory organs; at intervals these grooves 

 communicate with the exterior through pores or by little tubes 

 which run outwards through the scales (Fig. 79d). It is these 

 pores or modified tube-bearing scales which form the charac- 

 teristic external line, generally running from behind the head 

 to the base of the caudal fin, and not infrequently continued on 

 to the fin itself It may run more or less straight along the side 

 as in the Trout (Salmo) or Carp {Cyprinus) ; it may be curved 

 upwards to follow the line of the back as in the Perch (Perca) , 

 or downwards and parallel with the line of the belly as in the 

 Roach (Rutilus) or Bream [Abramis). In the Parrot-fishes 

 (Scaridae) and others (Fig. 44a) it is disconnected, the upper 

 portion ending abruptly below the soft dorsal fin, and the 

 lower portion commencing again below it and running back- 

 wards to the tail in the usual manner. In the Greenlings 

 (Hexagrammidae) of the North Pacific (Fig. 44b) there may be 

 several lines on each side of the upper part of the body. In the 

 Tongue Soles (Cynoglossidae) there may be one, two or three 

 lateral lines on the upper surface of the body and one, two or 

 none on the lower, in addition to a complicated system of lines 

 on the head (Fig. 44f). The continuation of the lateral line 

 system on to the head occurs in most fishes, but where it is 

 formed of deep-seated canals running along the surface of, or 

 even perforating the bones, it is externally quite inconspicu- 

 ous in this region. In many fishes, notably in the Flat-fishes 

 {Heterosomata) , the line runs straight from the tail to the tip 

 of the pectoral fin, and then forms a more or less well-defined 

 arch above the fin itself (Fig. 44e). In certain groups (Gobies, 

 Cyprinodonts) the surface structures of the lateral line are 

 entirely wanting. In the Sharks it is represented by a simple 

 groove protected by overlapping shagreen denticles. 



It is sometimes of considerable importance to be able to 

 determine the age of a particular fish, and especially of those 

 fishes which form the national food supply; this can be carried 

 out, in some fishes at least, by what is known as scale-reading. 



