INTRODUCTION TO THE FISHES 209 



arising from a more or less circular bony base, were called Placoid 

 scales ; the scales which have the free margin more or less comb- 

 shaped were termed Ctenoid ; and those marked with a concentric 

 structure were named Cycloid. To the two latter groups the great 

 multitude of living fishes belong, but in the earlier ages of the 

 earth's history Ganoid fishes were the prevailing types. 



The concentric rings observable on the surface of the scales 

 of many Teleostean (bony) fishes are an index to the age of the 

 fish. Scale growth is accelerated during the warmer season of the 

 year, but diminished during the colder season in such a way 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 the scale, 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. 



The external differences between the sexes are less conspicuous 

 in fishes than in birds and mammals, but are of frequent occurrence 

 and considerable interest. Among the characters peculiar to the 

 males may be distinguished organs usually of an intromittent type 

 concerned in the fertilisation of the ova and introduction of the 

 milt into the body of the female ; peculiarities of structure for 

 aiding the union of the sexes ; structures which may be displayed 

 during courtship, and weapons used for fighting rival males. In 

 all fishes except the Elasmobranchs and a few Teleosts the fertilisa- 

 tion of the eggs takes place in the water after their extrusion from 

 the female, the male depositing its seminal fluid over the eggs 

 or in the immediate vicinity. The number of eggs produced is 

 often enormous, as we may gather from the following figures given 

 by Dr. T. Wemyss Fulton : In a ling, 61 inches long and weigh- 

 ing 54 lb., the ovaries contained 28,361,000 eggs ; a turbot, 17 Ib. 

 in weight, had 9,161,000 eggs ; a cod, 2iJ lb. weight, had 6,652,000 

 eggs ; a flounder, i lb. 9^ oz. in weight, had 1,638,000 eggs ; 

 while four herrings had respectively 30,000, 26,000, 47,000, and 

 o 



