AIR-BREATHING IN FISHES 39 



15 m. but no higher; for its compensatory movements are 

 insufficient to prevent it being taken up to the surface because 

 of the expansion of its swimbladder. It may rise further, of 

 course, but this must be done slowly because of the slow rate 

 of the processes reducing the swimbladder volume. Studies of 

 the vertical migration of fishes using echo-location have shown 

 the presence of the so-called deep scattering layer which moves 

 up and down in a diurnal rhythm. The cause of this layer is in- 

 completely understood but many experts consider it to be due to 

 the reflection of the waves by the swimbladder. The swimbladder 

 is also important economically because when the catch of a 

 trawler is raised very rapidly to the surface the gas contained in 

 the swimbladders will expand and frequently it may burst and 

 cause sufficient damage to the catch to render it a financial loss. 

 The absence of swimbladders in deep-water fishes is readily 

 understood because they are subject to such enormous pressures 

 that secretion would need to be very great and would have such 

 a limiting effect upon their movements. Those which are habitu- 

 ally bottom-living do not require to be neutrally buoyant when 

 at rest. The difficulties of vertical migration could be overcome 

 if the swimbladder were made of steel and were prevented from 

 changing in volume as the fish swam towards the surface. It is 

 notable that some of the fishes of abyssal groups (e.g. Coela- 

 canths) had ossified swimbladders. And in the cartilaginous 

 fishes the problem is overcome by adjustments of the total 

 density of the fish resulting from the presence in the liver of fats 

 of very low specific gravity. 



(d) RESPIRATION OF LUNGFISHES (DIPNOI) 



Lungfishes arose in the Devonian period and have a long 

 fossil history. They are closely related to the group of fishes 

 (Crossopterygii) which gave rise to the first land vertebrates. In 

 common with crossopterygian fishes they possess internal 

 nostrils which can be seen in the three living genera of lung- 

 fishes. These are found in the three southern continents and are 

 Neoceratodus (Australia), Protopterus (Africa), and Lepidosiren 

 (South America). As their common name suggests, all three 



