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The Periodic Growth of Scales in Gadidae and 

 Pleuronectidse as an Index of Age. 



By 



J. Stuart Thomson, 



Lecturer on Biology, Municipal Technical Schools, Plymouth. 

 (With Plate V.) 



The first part of a lengthened and detailed statistical paper on the 

 structure and seasonal growth of Gadoid and Pleuronectoid scales will 

 shortly be issued from the Marine Laboratory. 



The forthcoming paper will show by means of tabulated statistics 

 that scale growth is accelerated during the warmer season of the year ; 

 but diminished during the colder season in such a methodic manner as 

 to cause the formation of annual rings. The formation of these annual 

 rings results from the fact that the lines of growth on the scale surface 

 are comparatively widely separated from one another in that portion of 

 the scale formed during the warmer season of the year ; but much less 

 widely separated in that part built up during the colder season. Thus 

 by following the arrangement of the lines of growth on scales, it is a 

 simple matter to observe the starting place of any year's growth by the 

 comparatively wide separation of the growth-lines at that portion of 

 the scale, and in this way the surfaces of scales appear mapped out by 

 annual rings. These annual rings supply us with an index as to the 

 age of the fish, and may be roughly compared to the rings in many 

 trees. The annual rings in the stems of trees are due to seasonal 

 nutritive conditions, and the rings on the scales of fishes are probably 

 the result of seasonal environmental conditions such as food, tempera- 

 ture, etc. In more detail, the alternate occurrence of comparatively 

 rapid and slow areas of growth in scales is probably the result of the 

 variations in food, temperature, etc., which are associated with the 

 alternation of summer and winter. For example, the abundant supply 

 of food (plankton, etc.) during the warmer season of the year probably 

 has much connection with the comparatively rapid growth of the scale 

 at that time as compared with the slow increase during the colder 

 season, when there is a decrease in the supply of food. 



These facts appear to possess both scientific and economic importance, 



