312 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 lke the Flat 
Fishes or Pleuronectidae, when not in motion by the exercise of 
their fins, 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. 
1 Moreau, op. cit. pp. 3, 4. 
