452 Dr. V. Fatio on the Variability of the 



bladder is not closed, can evidently react more or less against 

 slowly increasing differences of temperature or pressure, it is 

 none the less probable that important diversities in pressure, 

 and rapid or considerable changes of temperature, must have 

 much influence upon the gestures of the individual under 

 different conditions and in different seasons, and thereby more 

 or less upon its external form and appearance*. 



I may state, in passing, that in these two experiments we 

 had the opportunity of ascertaining, in a very striking man- 

 ner, that all the fishes heated (towards the end of January, 

 when they were pale in colour) rapidly acquired, as the tem- 

 perature increased, a much more brilliant coloration, somewhat 

 analogous to the nuptial livery. The bullheads, which were 

 at first whitish underneath, became almost black on the throat 

 and belly ; the perch and tench acquired very brilliant me- 

 tallic reflections ; the spirlins displayed a fine violet band at 

 the upper part of the flanks ; and the minnows already pre- 

 sented here and there on their lower surfaces the red colora- 

 tion specially characteristic of the season of amours. When 

 placed, after the experiment, in water at 9° or 10° C. (48°* 2 

 or 50° Fahr.), these very rapidly lost all their temporary bril- 

 liancy. 



Returning, now, to the consideration of fishes under normal 

 conditions or in freedom, I may remark, first of all, that the 

 species of the families with a mixed diet or omnivorous, and 

 with an air-bladder in communication with the exterior, have 

 always appeared to me more subject to vary, as to the form of 

 the buccal organs or organs of prehension, than the fishes of 

 exclusively animal or vegetable diet confined with them under 

 the same conditions. Elsewhere, in another medium, it might 

 be the latter that would vary most in this particular, or, per- 

 haps, some other part would be called upon to vary first of 

 all. A rule established upon such principles for one family 

 will necessarily always be subject to apparent exceptions in 

 another group. 



Among other things we shall very speedily remark that 

 the plan of the modifications of the buccal cleft varies, in fishes , 

 in diverse orders, even under similar conditions, according to 

 the kind of gymnastic lohich may be permitted by other organs, 

 such as the fins or the air-bladder. The smelt, which takes 



♦ It would be interestiug to investigate, by a thorough study of all the 

 gestures of the fish in diftereut circumstances and at diiferent seasons, 

 why sometimes a certain species has a more or less developed air-bladder, 

 whilst another, belonging to the same genus, is, on the contrary, desti- 

 tute of that organ. 



