Respiration and Metabolism 217 



A number of accessory respiratory adaptations are found, particularly among 

 fish, to increase the efficiency or to supplement the action of gills. Many of 

 these adaptations permit air breathing, allowing some obviously aquatic teleosts 

 to become temporary land dwellers. Thus the branchial chamber may become 

 adapted as a gas storage chamber to aerate the gills, as in the "jumping Hsh" 

 Periophthalmus.'-'^'^'^' •"" By increasing surface area and vascularization the 

 branchial chamber may provide for some gas exchange itself, as in the electric 

 eel, Electrophorns.*^^ Well developed branchial diverticula, facilitating gas 

 exchange, have been described for many air-breathing fish, as Anahas, Clarias, 

 and Periopthalmns^'^- -^^ (Fig. 39). These accessory respiratory structures not 

 only subserve aquatic gills but may become essential in providing adequate 

 oxygen, as in Anahas, which will drown if it is denied access to atmospheric 

 air. Buccal-phai:yngeal respiratory exchange among vertebrates is not limited 

 to fish; a considerable body of evidence points toward its occurrence in anu- 

 rans,"^*"'-' turtles, '-■^' -''■^ and lizards. '°^ 



Gill respiration in a sense represents a stage of cutaneous breathing in which 



Fig. 39. Branchial diverticula in the air-breathing teleost, Clarias lazera, subserving 

 the gills in aerial respiration. From Marlier."^ 



appendages have become specialized for gas exchange— structures which in- 

 crease the area and efficiency for gas transport across the respiratory surfaces. 

 They show considerable modification with changes in the oxygen environment 

 and frequently function in combination with other respiratory structures in 

 the exchange of vital gases. Being external structures without strong means of 

 support, they are most commonly adapted to the water environment. 



Lungs. These are most simply defined physiologically as respiratory sur- 

 faces folded into the body, a description that includes a variety of structures.-"' 

 They are usually aerial, although in some cases they are water-filled and 

 extract dissolved oxygen. In all cases the lung surfaces must be moist, and gas 

 transport occurs across a thin water film. Lungs are of two general types, 

 diffusion and ventilation, depending on the presence of renewal mechanisms. 

 All the lung-like organs considered here are parts of or outgrowths from the 

 alimentary tract. ^ 



Water-Lungs. Water-lungs are found in several invertebrate phyla and 



