146 Profs. T. W. Bridge and A. C. Haddon. [June 16, 



a means of pressure adjustment during ordinary locomotion, but un- 

 fortunately, in Moreau's experiments, the amount of gas previously 

 abstracted from the air-bladder, or the extent of the pressure varia- 

 tion to which the organ was exposed, was often so considerable that 

 from several hours to several days were required to restore the normal 

 equilibrium of the Fish. No attempt has yet been made to obtain 

 accurate measurements of the precise rate of secretion or absorption 

 under conditions involving relatively small variations of level and 

 pressure. Three important factors appear, however, to be well estab- 

 lished : (1) that gaseous secretion and absorption are relatively slow 

 processes in all Fishes ; (2) that, although retia mirabilia are not in- 

 dispensable to these processes, there can be no doubt that both take 

 place much more rapidly in Fishes that possess such structures than 

 in those in which they are wanting ; and (3) that increased hydro- 

 static pressure accelerates the rate of secretion, while diminished 

 pressure exerts a similar influence on absorption. 



In the case of the Physoclisti, which very generally possess retia 

 mirabilia, but no pneumatic duct, gaseous secretion and absorption 

 must be the only means of pressure adjustment ; but how far these 

 methods can be employed during the more or less rapid changes of 

 level that occur in ordinary locomotion is doubtful, and, bearing in 

 mind the relatively slow rate at which these processes take place, 

 even in Fishes with retia mirabilia, we incline to the opinion that 

 they are more likely to be of advantage {o the Fish during such 

 gradual changes of level as may occur in the course of diurnal, 

 seasonal, or other periodic migrations than in ordinary locomotion. 

 That this is the case seems to be suggested by many features in their 

 habits, which tend to prove that most Physoclisti have but a com- 

 paratively restricted vertical range in so far as normal locomotion is 

 concerned. 



The relatively few Physostomi which possess an air-bladder and no 

 ductus pneumaticus are in precisely the same position, as regards 

 their mode of pressure adjustment, as the majority of the Physoclisti ; 

 and of the remainder, we may consider, in the first place, the Ostario- 

 physesB, which possess not only an open pneumatic duct and a 

 Weberian mechanism but are without retia mirabilia. 



The absence of retia mirabilia in all hitherto investigated Ostario- 

 physece suggests that as a means of pressure adjustment gaseous 

 secretion and absorption are of minor importance. On the other 

 hand, Moreau's experiments prove that those Ostariophysese with 

 which he experimented possess a great advantage over the 

 Physoclisti in that they can, during ascent, more rapidly adjust the 

 volume of gas in the air-bladder to decreased hydrostatic pressure by 

 mechanically liberating a certain quantity of gas through the pneu- 

 matic duct than by relying solely on the absorptive capacity of the 



