SKIN, SCALES, AND SPINES 



[OI 



The discovery, made some years ago by Mr. H. W. Johnston, 

 that every Salmon carries its own hfe-history clearly written 

 on each one of its scales, has proved to be of incalculable value to 

 scientific men, and this method of age-det<ermination has since 

 been appUed to a number of other species, generally with success. 

 It has already been shown how the scale of a typical Bony 

 Fish develops in the dermis and gradually grows until it comes 

 to overlap the one lying immediately behind it (p. 90). This 



Fig. 44. LATERAL LINES. 



a. Parrot-fish {Scar us emblematiais),x^ ; b. Greenling (Hexagranifniis stelleri), 



X^; c. Bream (Abratnis brama),xi-; d. Viviparous Perch {Hysterocarpus 



traski), X i ; e. American Flounder {Paralichthys dentatus),xi ; f. Tongue Sole 



{Cy7ioglossus versicolor), X j. 



growth goes on throughout life, but seems to be retarded as the 

 fish becomes really old. As mentioned above, the number of 

 scales in any species remains constant throughout the life of the 

 fish, so that as the fish grows the scales must inevitably increase 

 more or less proportionally in size. Now, if the scales of a 

 Salmon be examined under a low-power microscope or hand 

 lens, it will be observed that each one is made up of a number 

 of rings arranged concentrically like the rings on a target. 

 Some of these rings are seen to be well separated, others closer 



