SCALE OBSERVATIONS OF THE SQUETEAGUE AND PIGFISH. 



3" 



INTERPRETATION OF THE RADII. 



Various conjectures as to the function and importance of the radii appear in the 

 literature on scales. They were an important item in the Agassiz-Mandl controversy. 

 In some systems of classifications by scales they have Deen considered constant enough 

 to be used in distinguishing genera and species. Observations recorded below point to 

 conclusions differing from any that have been advanced hitherto. 



In examining a large tarpon scale focussing was accomplished by bending the scale 

 on the stage. While this was being done the radii were noticed, their edges coming 

 closer together as the scale was bent (viewed from the distal side), and they seemed 

 to be lines of most flexibility. 



The number of radii on different parts of the body of the fish seems to vary as the 

 mobiUty of the parts. (Text fig. 2.) On the caudal peduncle more radii were found 

 than on any other part of the body. This 

 number decreases (subject to the influence of 

 size, shape, thickness, etc.) as one proceeds 

 anteriorly. On all the inflexible parts — i. c, 

 above the head, on the opercula, nape, etc. — 

 the scales were entirely without radii; but' 

 proceeding posteriorly, on the parts where 

 there is slight movement, a wave was found 

 in the anterior field. (PI. lii, fig. 4.) Pro- 

 ceeding farther posteriorly, the number of 

 radii increases and there is a tendency toward 

 an increase ventrally from the median line of 

 the back. In no case were radii found on 

 scales taken from inflexible parts of the body, 

 and the other factors being equal, notably 

 shape, their number varied directly as the 

 flexibility of the part from which the scales 

 were removed. 



These facts suggested that radii might 

 be hinges to permit the scale to bend in 

 adaptation to the movements of the body of 

 the fish. The fact that all degrees in the formation of radii, from total absence, then 

 wavy folds, then a few to finally numerous radii, are found and that these correspond 

 with the mobility of the part, which varies from zero, then slight, and finally to the 

 maximum on the peduncle, is alone sufficient evidence to support the hypothesis that 

 radii are simply hinges. 



There are numerous other evidences to support this hypothesis. As was shown 

 under the head of age determination, the uncalcified parts stain red with picrocar- 

 mine; the radii stain heavily with it. Plate Liii, figure 14, shows a cross section of a 

 scale that will illustrate how the radii facilitate bending. 



An examination of a scale mil make this clear. It will be noticed that the radii 

 do not begin at the focus, but the young scale must increase to a size that will interfere 

 with the movement of the fish, i. e., the scale must become stiffened by calcification 

 so as not to bend readily with the body of the fish before the radii begin to appear. 



Fig. 2. — Polygon showins the occurreuce of radii on scales 

 taken from different parts of the body. H, scales from 

 head; D, sea les from a point below the anterior base of 

 the spinous dorsal fin; P, scale from a point beneath 

 pectoral fin; C, scales from caudal pediuicle. 



