234 BULLETIN OF THE BUREAU OF FISHERIES. 



Fig. 12. Individual (Blon black and white background (i cm. squares) from July 31, 11.40 a. 51., 

 to August I, 9 a. m., after having been on (i sq. cm. circles) from July 30, 3. p. m., to July 31, 11.40 a. m. 

 Note that the dark areas are darker than in figure 10. 



Pl.we XXII. 



Fig. 13. Individual (B) on a smooth white background continuously from July 23, 2 p. m., to 

 July 25, 10 a. m. It was alternately on white and on black about equal time during the preceding 

 seven days. The skin actually appeared much whiter and less mottled and the animal less conspicu- 

 ous than the photograph indicates. This is partly due to the shadow in tlie background along the 

 ventral side. In specimens kept continuously on white for two months the skin became much more 

 imiformly white. 



Fig. 14. Individual (B)on black and white background (circles 5 mm. in diameter) from August 11, 

 12 m., to August 13, 1.30 p. m. Note the remarkable difference between the pattern in this figure and 

 that in figiu'es g and 13. 



Fig. 15. Individual (B)on black and white backgrotmd 2 cm. squares from August 24, 11 a.m., 

 to August 27, a. m., after having been adapted to white. Note that the light areas are much larger 

 than on figure 15, which shows the same specimen 011 the same background at a different time. A great 

 variation was observed in the size of these areas during the time the fish w as on tliis background. There 

 is some evidence indicating that the relative amount of black and white produced in the skin depends 

 upon whether tlie head is over a black square or over a white square. If it is over a white square there 

 is considerably more black near the eyes tlian white, and if it is over a black square the opposite is 

 true. While in otlier positions one eye may be influenced mainly by black and the otlier mainly by 

 white. This no doubt accounts for the variation mentioned. In other words, a fish 14 cm. long can 

 not simulate light and dark areas of this size as accurately as smaller ones, although larger specimens 

 can. 



Fig. 16. Individual (B) on black and white background (circles 5 mm. in diameter) from August 

 15, 10 a. m., to August 16, g a. m. This photograph sho^vs one of the most remarkable concealing pat- 

 terns observed. The fish appeared to contain numerous holes. 



Pl.-vte XXIII. 



Fig. 17. Individual (B) on black and white background (i sq. cm. circles) from August i, 4.30 

 p. m., to August 2, II a. m. Probably not fully adapted. 



Fig. 18. P. albiguttiis (E), 10 cm. long, on black and white background (i sq. cm. circles) from 

 August I, II a. m., to August 5, 9.40 a. m. On this background the specimen was nearly uniformly 

 gray much of the time, and the simulation of the figures in this background was at no time as good as 

 it was in larger specimens, although on the finer-grained backgrounds it was quite good, showing tliat 

 the maximum area of figiu'es successfully simulated depends upon the size of the fish. 



Fig. 19. P. albiguttus, individual (C), 14 cm. long, on black and white backgroiuid (i cm. squares) 

 from July 25, 2 p. m., to July 28, 10.30 a. m. Note that the pattern is similar to that in individual (B), 

 figure 12, although specific individual characteristics can readily be found. Compare, e. g., the ventral 

 I icellus in these two figures. 



Fig. 20. P. albiguttus (E), 1.0 cm. long, on black and white background (5 mm. squares) from 

 August 6, 12.30 p. m., to August 8, 10 a. m. 



Plate XXIV. — Effect of mechanical stimulation on the pattern produced in the fish. 



Fig. 21. p. albiguttus, 16 cm. long, in shallow bluish-gray mottled granite pan from August i to 

 August 4, 10.30 a. m. Excellent simulation of the background on the entire stu-face except the three 

 ocelli, which are nearly black. 



Fig. 22. Same specimen photographed in the granite pan, August 5, 4 p. m., very shortly after 

 mechanical stimulation. Note the contrast due to the enlargement of the regions around the ocelli and 

 the appearance of numerous dark and light spots. These regions and spots were considerably more 



