COLOR CHANGES AND ADAPTATION IN FISHES. 191 



of the same area in the different photographs mentioned will show. In all of the 

 specimens of this species there are three small but prominent black areas, ocelli, which 

 stand out rather conspicuously, especially in animals adapted to a background con- 

 taining relatively little black. They are well shown in some of the figures on plates 

 XXVI and xxxii, and also in many other figures. In individuals adapted to a white 

 background they are light gray and quite indistinct, but they can still be seen. On 

 backgrounds containing much black, however, these areas become continuous, with 

 larger black patches, and can no longer be distinguished. 



Without a lens it can be readily seen that the ventral one of these ocelli is elliptical 

 in outline in figure 9, and in all of the other photographs of this individual in which 

 it can be seen at all (fig. 9-17, 23-29, 49-55). In figure 19, which represents a different 

 individual, this area is, however, very nearly circular, and the posterior border contains 

 a small but conspicuous white spot. These characteristics are seen in all ot the photo- 

 graphs of this individual. They are particularly prominent in figures 33 and 34. In 

 figures 31 and 32, photographs of still a different individual, this area has other dis- 

 tinguishing features. Thus any individual could be recognized by the characteristics 

 of this area alone. There is probably quite as much individual difference in every other 

 section of the surface, so that the patterns of different individuals of the same species 

 which superficially appear so nearly alike are, in detail, so different that every indi- 

 vidual could be recognized by a thorough examination of any small area of the pig- 

 mented surface probably not larger than 0.5 mm. square. This individual variation 

 becomes very evident in a comparison of figures 36 and 37 with figures 39 and 40; the 

 former represent enlargements of a given area of one individual on two different back- 

 grounds, the latter enlargements of the same area of another individual of the same 

 species, P. albiguttus, on similar backgrounds. By examining these figures closely it 

 will be seen that while nearly all of the light and the dark areas and spots found in one 

 individual are also found in the other, they differ considerably in form. There is, in 

 fact, more difference in the details of the patterns in the two individuals on the same 

 background (granite pan) than there is in the patterns of the same individuals on the 

 different backgrounds. 



Effect of mechanical and other stimuli on the pattern. — If Paralichthys or Ancylopsetia 

 is mechanically stimulated, contrastive patches suddenly appear in the skin. If the fish 

 are adapted to a dark background, numerous white spots appear, and if adapted to a 

 white background numerous black spots appear. In either case the animal becomes very 

 conspicuous. This is especially marked in Paralichthys. (Fig. 21, 22.) The degree of 

 stimulation required to produce the reaction varies greatly. In animals that have not 

 been handled much it ordinarily requires only a light touch. Strange objects brought 

 in the field of vision also induce this reaction. Thus, to cause the reaction, it is often 

 only necessary to bend over the aquarium or to bring a strange specimen within a dis- 

 tance of 15 to 30 cm. In such cases the change in the chromatophores is clearly due to 

 stimuli received through the eyes. Similar changes usually occur during the process 

 of feeding. This is particularly marked in specimens adapted to white. No new spots 

 originate in any of these cases. That is, the spots, both black and white, appear in 

 the same relative regions in all specimens and in the same regions in which they appear 

 when they are caused by any other stimulating agents. As to the biological significance 

 of these phenomena, if there really is any, we are quite in the dark. 



