116 



ANIMAL BIOLOGY 



above the freezing point. In such surroundings some possess 

 luminous organs, some have immense eyes to make the most 

 of little light, some are blind and so depend upon tactile organs, 

 some have an immense mouth and an enormous stomach capa- 



Fig. 74. -- A, a Flying-fish; B, a deep-sea Fish. (From Gunther and Lull.) 



ble of digesting a fish nearly their own size, and so on and on — 

 adaptations seemingly endless — a Fish for every condition of life 

 in water. (Figs. 72-74.) 



3. Lung-fishes 



Finally, mention must be made of a group that flourished in 

 the geological past but is represented to-day by only five fresh- 

 water species. They are known as Lung-fishes, or Dipnoans, 













Fig. 75. — African Lung-fish, Profoplerus anneclens. (From Dean.) 



because an air sac, which in most other Fishes acts as a hydrostatic 

 organ, here opens into the pharynx and functions as a lung when 

 sufficient water is lacking during a drought. Lung-fishes have been 

 regarded as intermediate between Fishes and Amphibians, but 

 they have many structures that are similar to those of the lower 

 Fishes. They are an interesting and puzzling remnant. (Fig. 75.) 



