40 VERTEBRATE RESPIRATION 



genera have a diverticulum of the pharynx (bilobed in Protop- 

 terus and Lepidosiren), which can be used to breathe air. The 

 order of the three genera given above is one of increasing abiUty 

 to breathe air and a decreasing abihty to survive aquatic con- 

 ditions. Only Neoceratodus is able to survive if it is not allowed 

 to come to the surface and only this genus has well-developed 

 gill filaments. Also correlated with their different abilities with 

 respect to aerial and aquatic respiration are differences in the 

 circulation to the branchial region. The branchial arches of 

 Lepidosiren pass through the gill arches with few capillaries and 

 the blood does not come into close contact with water. In 

 Neoceratodus the branchial arches break up into fine capillaries 

 in the gills. It normally breathes under water with the mouth 

 slightly open and a current produced by slow movements of the 

 two opercula. This Australian lungfish rarely comes to the 

 surface, but the other two genera regularly (e.g. every 15-30 

 minutes in P rot op terus) come to the surface and gulp air through 

 the mouth. Notwithstanding the presence of internal nostrils 

 in the Dipnoi, it appears that air is always taken in at the mouth 

 in those living fishes which breathe air. It might be supposed 

 that air enters through the nostrils during periods of drought. 

 In fact only Protopterus makes a mud cocoon although Lepido- 

 siren is also able to aestivate, but Neoceratodus cannot survive 

 out of water for long periods. In Protopterus there is a well- 

 defined plug of mucus in the nostrils and the cocoon only com- 

 municates with the air through a tube which enters the mouth. 

 In this case, then, there is again no doubt that air-breathing 

 does not take place through the nostrils. 



Living in a cocoon during periods of drought is a very good 

 adaptation where such periods are lengthy and Protopterus may 

 survive as long as 3-5 years in such a state. As long as the 

 cocoon remains intact the fish is protected against desiccation. 

 Its entry into aestivation is a well-defined response to conditions 

 where the water level falls and the habitat becomes muddy. The 

 fish dives into the ooze and turns upwards so that its snout lies 

 just below the surface of the water. As the water level falls still 

 further the fish makes occasional excursions to the surface and 



