THE SCALES OF THE SALMON ID^E 251 



appearance common to the salmon, while the anterior 

 edge of the trout's tail will always present a straight edge 

 when the tail is opened. Secondly, the adipose fin of the 

 parr is always without pink colour, while the trout's adipose 

 fin has always a more or less pink spot or colouring. It is 

 not until the smolt stage of the parr approaches that a 

 difference is first noticed in the appearance of the skin, when, 

 either from a changing physiological condition in the young 

 salmon, or from a greater absorption of the material elements 

 of which the skeleton and exo-skeleton are built, fresh 

 laminae more rapidly succeed each other, and as each 

 fresh addition is added to the young scale its superficial area 

 is increased by the width of the latest formed band of scale 

 material (see Plate XXVIIIa). 



The scales thus expand, and become imbricated at their 

 posterior ends. This overlapping does not stop the growth 

 of this portion of the scale, however, for the ducts which 

 must supply the material from which the scale is formed 

 still continue their functions. 



The scale, as it grows, now presents the appearance of 

 a delicate silvery armour, which characterizes the advent of 

 the smolt period, and so long as the smolt, grilse or salmon 

 is healthy and in good condition, this silvery sheen will be 

 retained by the exposed portion of its scales. 



The brilliant opalescent glaze of the scale is transmitted 

 by some iridescent pigment in the membranous covering, 

 possibly of a calcareous nature, which, if examined under a 

 magnifying glass, will recall the iridescent lustre often 

 seen in the track of snails. The formation of the 

 membranous covering of the scales, in which this brilliant 

 sheen occurs, appears to arise from a mucous secretion 

 issuing from a similar class of ducts to those which supply 

 the material creating the shells of certain of the mollusc 

 family. 



