Chap. 33 VERTEBRATES — LOWER CHORDATES AND FISHES 671 



salmon, trout, and others, they are cycloid, more or less circular and smooth- 

 edged. 



As the fish grows, scales increase in size but not in number. Within close 

 range, each individual has the same number as others of the same species. Each 

 scale enlarges by the addition of many bands or rings per year (Fig. 33.10). 

 The width of a ring signifies the rate of growth and is based on the metabolic 

 activity of the fish. In summer, when food is abundant, the bands are broad 

 and the lines farther apart. In winter, food is sparse and growth is slow; the 

 lines are close together. The age of many, though not all, bony fishes can be 

 told by the number of summers and winters recorded. It is believed that most 

 fishes grow as long as they live and usually obtain enough food to have some- 

 thing extra beyond routine upkeep. 



The color of skin is due to saclike cells, the chromatophores, that contain 

 pigment. They are distributed in great numbers through the deeper layer 

 (dermis) of the skin. Each contains only one color, usually red, orange, yel- 

 low, or black and these pigments may be spread out in the cells or con- 

 tracted to pinpoints. White, blue, and green are due mainly to the break up of 

 light rays on the surfaces of crystals of guanin that are colorless metabolic 



Fig. 33.9. Early stages of eastern brook trout, Salveliniis fontinalis. A, eyed 

 eggs showing the embryos through the egg envelopes; B, hatching; C, a group of 

 free swimming fry; and D, a recently hatched fry with its blood vessels outspread 

 through the yolk-sac (enlarged about five times). (Courtesy, Needham: Trout 

 Streams. Ithaca, N. Y., Comstock Publishing Co., 1940.) 



