100 THE rERIODIC GROWTH OF SCALES IN GADID^ 



them much more regular in their arrangement than the scales of 

 whiting captured at sea. The lines of growth appeared almost uni- 

 formly separated from one another, and because of this I could not 

 observe any distinction into sunnner and winter areas such as are 

 marked out in my plates. 



Another noteworthy point about the lines of growth in the scales of 

 this whiting was that they appeared throughout to be closer to one 

 another than is the case in captured fish. This would probably indicate 

 a uniformly slower growth of the scale. 



The temperature of the water in the Plymouth tanks remains fairly 

 constant ; but there is naturally a distinct difference between the 

 summer and winter temperature, and the whiting in question may be 

 taken as having been fairly regularly supplied with food. From these 

 facts, and also from the fact that fish from deep water, where the 

 temperature of the sea does not show marked variation in summer 

 and winter, show annual rings as clearly as those from shallow water 

 where there is a marked difference between the summer and winter 

 temperature, inclines me to believe that it is a question of variation 

 in the food-supply rather than variation in temperature which in- 

 fluences the metabolism of the fish, and indirectly brings about the 

 formation of annual rings in scales. 



The scales of this aquarium whiting showed, however, some interesting 

 points, firstly as to the number of lines of growth : the total number of 

 these lines was on an average 50, and whiting from the sea which I 

 determined to be of about the same age, though of a larger size (see 

 tables), showed on an average 43 lines of growth. It appears to me, 

 if 1 had not already known the real age of this captive whiting, that 

 from my tables of calculated ages for captured whiting I would at 

 least have arrived at the approximate age by counting the number of 

 lines of growth in the scales. 



In regard to the sizes of scales in this captive whiting, they were 

 on an average the following: Total length of scale, 2*00 mm. ; maximum 

 breadth of scale, l"50nim, ; long axis AB", llO mm. On comparing 

 the figures above with those given in my tables, it seems that the size 

 of the scale is small for the number of growth lines present, and this 

 one might expect from my previous observation that the growth lines 

 are all uniformly closely adjacent to one another. 



