AIR-BREATHING IN FISHES 41 



this leads to the hollowing out of a chamber which opens to the 

 air through a small blow-hole. Finally, the water drains away 

 completely and the fish is able to breathe without coming to the 

 surface. It curls up with its tail above the head and covering the 

 eyes. Mucus covering the body dries into a hard cocoon which 

 only opens to the outside through a funnel which extends into 

 the mouth. The wall of the cocoon has lipo-protein water- 

 proofing layers similar to those of the insect egg. 



During aestivation the animal is unable to feed and there are 

 some interesting adaptations in its metabolism. These have been 

 investigated by keeping African lungfish in the laboratory with- 

 out food for varying periods. During such fasting the oxygen 

 consumption of the animal falls gradually to levels which are 

 determined by the time since it last took food. Similarly within 

 the cocoon the O2 consumption falls by 50% during the first 

 week and finally after several months to only 10% of its initial 

 level. Correlated with the reduced metabolism Protopterus 

 breathes only once every hour or two and its heart rate is only 

 3/minute. If the cocoon is carefully removed the fish is found to 

 be moist inside and if kept in a suitable chamber for several days 

 it may remain motionless, except for the infrequent respiratory 

 gasps. Awakening from this remarkable state of dormancy is 

 readily induced by the threat of asphyxiation. Unlike other 

 fishes the entry of water to the mouth results in attempts to 

 breathe air and finally convulsive jerks as it awakens. After 

 awakening, there is a rapid increase in body weight before the 

 animal feeds and this is due to the intake of water. 



