3 1 2 FISHES CHAP. XI 



can more readily adapt themselves to the diminished pressure of a 

 ■higher level by ejecting the needful amount of gas than by relying 

 upon the process of gaseous absorption/ This conclusion is in 

 harmony with the results of experiment and with much that is 

 known of the habits of these Fishes and their greater freedom of 

 locomotion in the vertical direction. 



These briefly summarised conclusions as to the hydrostatic 

 function of the air-bladder must, however, be accepted only in a 

 general sense. There are many structural anomalies in the air- 

 bladder of Fishes which are very difficult to explain, or to corre- 

 late with any variations in the habits or in the locomotor activities 

 of its possessor. 



In this connexion it may be mentioned that the presence or 

 absence of an air-bladder in different Fishes seems to some extent 

 to be governed by two causes. First, whenever the requirements 

 of a Fish necessitate exceptional freedom of locomotion in all 

 directions the restrictions imposed by the presence of an air- 

 bladder are removed by its partial or complete suppression ; 

 a result produced, secondly, by the assumption of a bottom feeding 

 or ground habit on the part of the Fish. Fishes like the Flat 

 Fishes or Pleuronectidae, when not in motion by the exercise of 

 their iins, habitually rest on the sea-bottom, and, as an air-bladder 

 is useless under such conditions, it has, in consequence, undergone 

 complete atrophy. Not a few Siluridae, and some Cyprinidae, 

 inhabit the comparatively shallow waters of rapidly flowing moun- 

 tain torrents, and are often provided with suckers for attachment 

 to stones or rocks. To such Fishes as these a hydrostatic organ 

 is obviously useless, and it has hence become greatly reduced in 

 size, and in other respects approaches the condition of a vestigial 

 organ. 



^ Moreau, op. cit. pp. 3, 4. 



