CHAPTER X. 



THE RESPIRATORY SYSTEIVI. 



In describing the blood and integument we have ah-eady con- 

 sidered one of the means whereby respiratory needs are supplied. 

 Depending upon the environment of the animal in question, there 

 are certain other arrangements of tissues to accommodate gaseous 

 exchanges required by metabolism, 



THE RESPIRATORY SYSTEM OF FISHES. 



In the case of fish, respiration is effected mainly by means of 

 gills, which are specialized structures arising from the pharyngeal 

 region to accommodate respiration in water. Water enters through 

 the mouth, passes into chambers surrounding the gills, and bathes 

 them before passing outward through external gill slits. A typical 

 gill is composed of a median septum supporting two lamellae of 

 connective tissue and muscle and an exposed epithelial surface. 

 The septa are supported by cartilaginous or bony arches. The 

 lamellar surface is usually much folded and near it is an extensive 

 capillary supply bringing the blood close to the water for the 

 exchange of gases through the epithelial membranes. 



The air sac or swim bladder appearing in the majority of the 

 higher fishes is formed as a thin-walled pouch from the esophageal 

 region and is lined with thin e])ithelial mem})ranes. It has been 

 considered by some as being associated with respiration and also 

 as a hydrostatic organ. 



THE RESPIRATORY SYSTEM OF AMPHIBIANS. 



In amphibian larvjie and in adults of all mature water- 

 dwelling forms, thin membranous external gills are develo])ed as 

 epidermal folds. (Fig. 89.) They are lost in the adults of those 

 species adapted to terrestrial life. These membranes are well 

 supplied with blood vessels (Fig. 90) and ser^'e for respiration during 

 aquatic life. The skin is also vascularly e(iuii)i)ed and takes ])art 

 in respiration in many forms. 



