132 THE WEALTH OF THE SEA 



minnows, for the male is often not much more than 

 half the length of the female. 



Animals living on land generally are covered with 

 hair or feathers. Fishes, however, are, with some ex- 

 ceptions, covered with scales. Scales in fish are very 

 diverse. The tarpon, for example, has very large 

 scales, whereas the cod has small ones. Scales gen- 

 erally are roundish. The common eel, however, has 

 elongate and somewhat rectangular ones, partly 

 buried in the skin. Scales are very helpful to the 

 student in classifying fish and are also very useful 

 in determining the age of a fish, for they bear mark- 

 ings which are usually significant. In determining 

 the age of a fish special attention is given to the 

 rings in the scales, for these form very close to- 

 gether when growth proceeds slowly, and when 

 growth is rapid they form further apart. Therefore, 

 during winter, when the weather is cold and digestion 

 proceeds slowly — for the fish is a cold-blooded ani- 

 mal — little growth is gained, and the rings become 

 very crowded. Similarly, during the summer when 

 foods are plentiful and the weather is warm, diges- 

 tion is much more rapid, and growth is quickly 

 made, the rings form much further apart. Although 

 the study of the scale for determining the age of 

 fish is still quite new, it is now possible to state the 

 age of a given specimen of several different species 

 with a considerable degree of certainty. 



The chief value of the scale to the fish is for pro- 

 tection, and those that have no scales generally have 

 other means of protection. The catfishes, for exam- 

 ple, have large fin spines which bear barbs and are 



