ADJUSTMENT OF FLATFISHES 435 



commonly attained within one or two days at the most, after 

 which no further change was manifested. The fact already pointed 

 out by Pouchet^''^ and Van Rynberk" that 'practice' or habitua- 

 tion to these changes, greatly reduces the time required, was clearly 

 shown in my experiments with Rhomboidichthys. Certain speci- 

 mens, after several changes of background, were found to adapt 

 themselves in almost full measure to one of these within a frac- 

 tion of a minute. The first of these changes had required some 

 hours. 



A case worth citing here is that of specimen 11, which had been 

 kept for two months upon white and pale gray backgrounds. 

 This fish, after transfer to the pure black magnetite sand, did not 

 reach a medium shade in less than five days,^* and continued to 

 become darker for some days thereafter. Now this, it must be 

 remembered, was a change toward what would ordinarily have 

 been the normal shade. On the other hand, return to a pale gray 

 bottom resulted in the fish becoming appreciably paler within 

 an hour, and decidedly pale in the course of a day, although this 

 change was away from the condition normal to ordinary speci- 

 mens. 



Direction of the stimulus 



Before considering the question as to what part of the visual 

 field was most effective in calling forth these transformations, a 

 few words are necessary regarding the eyes of Rhomboidichthj^s. 

 These are extremely protuberant, being virtually mounted on 

 the ends of stalks like those of many Crustacea. They are extremely 

 motile, following moving objects in a rather uncanny fashion, or 

 even turning without any apparent stimulus. During these move- 

 ments the eye-stalks undergo an axial rotation, working together 

 in such a manner that the two eyes nearly always look in exactly 



'8 Op. cit., p. 73. 



"Op. cit., p. 549. 



38 A. Agassiz (Bull. Mus. Como. Zoology, 1892) records the production of "per- 

 manent albinos" in Gasterosteus, as a result of keeping these fishes on a white bot- 

 tom. The time required for such a change is not stated, however, and no evidence 

 is offered to prove that the alteration really was permanent. 



