530 



VERTEBRATE LIFE AND ORGANIZATION 



Opercalar 

 -chamber 



Visceral arch- 



Gill raKer 



Oxygenated blood leaving 



gills 



Unoxytfenated Hood 



^ .^ gills 



Gill rakers 



Moulh 

 cavity 



Gill slit— y^ '^^ M^ ^ , 



P . „ _Jm Ph ary nx»\^ Opercular 



Gill 



r, 1 i '^^ HscpK- ^-^chamber 



rj^''T//\jVBody wall 

 ^Coeloru 



c 



^-Qralvalve. 



D 



Figure 26.6. External respiration in fishes. A, The operculum has been cut away 

 to slidw tlie gills in the gill chamber. C and D, frontal sections through the mouth 

 and pliaiynx in the plane of Une a-b in the preceding figure. Water is entering the 

 pharynx in C; leaving in D. B, An enlargement of one of the gill sections shown in 

 C and D. (Modified after Storer.) 



and they supplement gill respiration by occasionally gulping air. There 

 is more oxygen in the air than in water and it can be extracted from 

 the air by gills so long as they remain moist. Closing the opercular 

 chamber enables the mudskipper to keep its gills moist for a while, and 

 even to come out onto the land. The European loach swallows air 

 and extracts the oxygen in a special chamber of its intestine! Other 

 fishes have vascular outgrowths from various parts of the pharynx or 

 opercular chamber that serve as accessory respiratory organs. Seem- 

 ingly the development of lungs, which are ventral outgrowths from 

 the pharynx, by early bony fishes was jtist one of many adaptations 

 which have evolved to supplement aquatic respiration. 



Extracting oxygen from swamp water, which is probably the en- 

 vironment in which lungs evolved, poses the problem of saturating 

 the blood with oxygen in an environment with a low oxygen and high 

 carbon dioxide content. As we explained in Chapter 5, the presence 

 of carbon dioxide reduces the oxygen-carrying capacity of hemoglobin. 

 The chemical properties of the hemoglobin of contemporary swamp 

 fish have changed in such a way that it can take up more oxygen in the 

 presence of a given amount of carbon dioxide. This change must also 

 have occurred dtiring the evolution of terrestrial vertebrates, for the 

 carbon dioxide content of the lungs is always higher than that of 

 the external environment, though, of course, not as high as that in the 

 tissues. 



