TELEOSTEAN FISHES 



351 





scales — by the addition of new bone especially round their edges — the 

 total number of the scales remaining constant. 



In the case of teleostean fishes inhabiting regions with well-marked 

 differences between the seasons there is 

 frequently evidence of the growth of 

 fresh bone being affected by these differ- 

 ences. During the winter, when meta- 

 bolism is less active, growth is slower 

 and the calcareous matrix of the bone 

 formed is more dense ; during summer 

 growth is more rapid and the bony 

 material is less dense. This periodicity 

 affects the scales as well as the bones in 

 general. In many cases the surface of 

 the scale projects in the form of fine 

 ■concentric ridges (Fig. 147, B) or other 

 pattern, and in the portions of the 

 scale formed during the summer these 

 ridges are more widely spaced out (5) 

 while in those formed during the 

 winter they are more closely crowded 

 together (W). As a consequence such 

 scales when examined under the micro- 

 scope are seen to bear a record of 

 their age, somewhat analogous to that 

 afforded by the annual rings of a tree- 

 trunk, and these scale records are made 

 practical use of in fishery investigations, 

 more especially in the case of fresh- 

 water fish of cold temperate climates 

 such as the Salmon. The record con- 

 veys other information than the mere 

 age. Where the act of spawning brings 

 about a great diminution in the bulk of 

 the body this is apt to cause the edges 

 of the scales to project and become 

 frayed and worn, and on the return to 

 normal conditions when the growth of the scale is again active the 

 new lines of growth follow the worn and irregular outline which remains 

 registered in the scale as a more or less distinct " spawning mark " 

 (Fig. 147; B, s.m). 



B 



Fig. 146. 



The tails of fishes. A, Ceratodus 

 (Dipnoi) ; B, Scylliutn, and C, A canthias 

 (Elasmobranchii) ; D, Cladoselache (fossil 

 Elasmobranch) ; E, Trout, and F, Mackerel 

 (Teleostei). 



