Larva of Chaoborus Crystallinus ( de Geer). 
355 
with the hypotheses put forward by those who have investigated 
the causes of maintaining buoyance of the Gorethra at various depths. 
A fish living near the surface, if brought down to a depth of, 
say, 10 metres, the air in its bladder becomes compressed to one- 
half its former volume, the buoyancy of the fish is correspondingly 
diminished, and it will sink ; but if it remains for some time at the 
depth of 10 metres it regains its equilibrium ( 9 ). 
This adjustment, however, only occurs after a lapse of several 
hours. 
The method of secreting a suitable amount of gas into the 
bladder to bring this about is described by Woodland, and we 
extract the following details from his paper : — 
“ The oxyhaemoglobin is the main source from which the 
oxygen glands of Teleostean fishes obtain supplies of oxygen in 
solution, and the cells of these glands cause the dissolved gases to 
appear in a gaseous form as bubbles in the cytoplasm, the cells 
Fig. 27. — Optical section of intake and outlet valve of air sac, showing 
tube passing through centre, with small round bodies occurring at 
intervals. Shaded part represents gas. 
subjecting these bubbles to greater pressure than that existing in 
the bladder. When the gas bubbles are expelled into the bladder 
lumen, and experience a diminution of pressure, they burst. 
“ Here we have the gas gland cells actually pumping the 
oxygen into the bladder. These are the conditions that arise with 
the descent of the fish into deep water. On ascending, the excess 
oxygen is rapidly absorbed by a body named the Oval, and passed 
back again into the blood. 
“ The contents of the bladders have been analyzed and found 
to contain — in varying quantities, according to species — oxygen, 
nitrogen, carbon dioxide, and argon. Oxygen, however, is the 
only gas that can be rapidly produced, hence the excess of this 
when the fish is subjected to increased pressure. 
“ In the bladders of Cyprinoids, and many other fresh-water 
fish, which mostly contain nitrogen, the gas glands are absent, 
the ordinary squamous epithelial lining here being capable, without 
undergoing any special modification into a gland, of pumping in 
the nitrogen alone required. There is no need of an oxygen gas 
