104 How Animals Changed 



ing adaptations for respiration in water and air, animals often 

 retained the former largely for excreting carbon dioxide and the 

 latter for acquiring oxygen. 



Aquatic animals in the past have become adapted many times to 

 air-breathing, in or out of water. This has apparently happened 

 most often in marshes and swamps where there was a lack of oxygen 

 and along ocean beaches where tidal rhythms frequently left animals 

 exposed. Fishes may have the swim-bladder adapted for breathing 

 air. In gars (Lepisosteus) , bowfins (Amiatus) , and some other 

 ganoids the swim-bladder serves as an important organ of respira- 

 tion. Such fishes are able to live in stagnant water where oxygen is 

 low by gulping air from the surface (Potter, 1927). In a genus 

 (Nemachilus) of Indian loaches the swim-bladder varies in differ- 

 ent species in accordance with the need of it for respiration. "In 

 swift currents it is greatly reduced and enclosed in bone; in deep 

 waters of lakes and at high altitudes it is re-developed" (Hora, 

 1930a). Morris (1892) believed that the swim-bladder of fishes 

 was first developed for respiration and in teleosts secondarily took 

 on hydrostatic functions. The gills of fishes, though often covered 

 by opercula, are in a sense "open" organs of respiration, but swim- 

 bladders, lungs, and other similar organs for air-breathing are in a 

 sense "closed" as they enclose internal cavities (Powers et al., 1932) . 



Fishes have become adapted to air-breathing in diverse and pe- 

 culiar ways. Gobies which skip about on muddy ocean beaches have 

 the adductors of the gill arches poorly developed; the epithelia of 

 their gill filaments show more mucous, cornified, and albumen- 

 secreting cells, and thus desiccation is inhibited; gill surfaces are 

 reduced; the skin, buccal, and branchial epithelia may serve for res- 

 piration; in some species there is a cavity above the first gill; the 

 mouth is small; the opercular aperture is narrow (Schottle, 1931). 

 Young land gobies have gills like aquatic gobies, but these change 

 with other features during metamorphosis. In modern gobies and 

 blennies "skin respiration is improved by the penetration of capil- 



