14 SALT-WATEH FISHES 



in the conger {Conger), or else replaced by rough tubercles, 

 as in the turbot {^Rhombus). Scales are so familiar to all who 

 have handled fish that they need little description and appear 

 to us, when viewed casually, as horny, semi-transparent growths, 

 generally covered with slime while the fish is alive, and over- 

 lapping each other like the feathers on the back of water-fowl. 

 These scales exhibit in point of size a range almost as great 

 as the fishes themselves, and between the scale of a tarpon 

 and that of a fresh-water eel there is perhaps an even greater 

 diflference in measurement than between their owners. 



When scales are viewed under a low-power microscope, 

 they exhibit certain structural differences. Those which have 

 the hinder free edge serrated are called " ctenoid " (^i.e. comb- 

 like), while those with the hinder edge smooth are known as 

 " cycloid." It would also seem as if there is a regular forma- 

 tion of annual rings in the scales of bony fishes, or at any rate 

 in those of the related cod and flat-fish families, which may, 

 when further studied, give a rough clue to the age of the 

 fish, just as the bark-rings give the age of a tree, and they 

 may be further compared on the ground of the similar in- 

 fluence of abundant warmth and nourishment, their growth 

 being accelerated in the summer and retarded in the winter 

 months.* 



The plates, or bucklers, which cover the body of the 

 sturgeon are distinct from ordinary scales, and are partly 

 composed of bone ; and the same may be said of the rough 

 tubercles on the turbot. Normally, these occur on the coloured, 

 or upper, surface only ; but numerous turbot reach our markets 

 from the coast of Norway in which tubercles are frequently 

 found on the uncoloured side as well, and this apparently 

 without reference to abnormal colouring. In sharks the so- 

 called scales are quite distinct from those of bony fishes, and 

 it is questionable whether they should be described as scales 



* See J. Stuart Thomson on " The Periodic Growth of Scales ... as 

 an Index of Age " {Jour. Mar. Biol. Assoc, January, 1902). 



