V 



EMERGENCY RESPIRATION 

 THE TRANSITION TO AIR-BREATHING 



In many localities, especially in the tropics, fresh-water ani- 

 mals are occasionally or regularly exposed to oxygen lack. 

 For marine animals corresponding experiences are very rare, 

 but the animals of the tidal zone have to adapt themselves to 

 the fact that the water itself is withdrawn for periods of some 

 hours or longer. In both emergencies air-breathing may be 

 resorted to, but as a stimulus to evolution lack of oxygen is by 

 far the more important. As organs for aerial respiration, gills 

 are on the whole rather poor structures, and the chief reason 

 is their softness, which causes them to collapse completely in 

 air and present only a greatly reduced surface. 



Certain animals living in the tidal zone have sufficiently 

 rigid gills to utilize them more or less successfully for air- 

 breathing. The opistobranch snail Ancula, which moves about 

 above and below the water line, possesses quite rigid branched 

 gills on its back. Several of the shore crabs (Grapsus, Carcinus, 

 according to Raffy, 1935) have gills which are sufficiently 

 rigid to allow them to obtain most of the oxygen needed from 

 the air, and the large pagurid Birgus latro has adopted an 

 almost completely terrestrial existence, even climbing trees. 

 The gill cavities are incompletely divided by a septum, and 

 the upper chamber, which always contains air, is provided 

 with a- large number of rigid bush-like structures serving the 

 aerial respiration (Semper, 1878). Several Brachyura (Gecar- 

 cinus) also live on land and breathe in a similar way. 



In all these forms mechanisms are found for retaining some 

 water to keep the delicate structures moist, and Birgo is re- 

 ported to take to the water at intervals of about a day. 



Even certain fishes belonging to the Gobiida and inhabiting 

 tidal mangrove swamps in the tropics have become adapted 



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