148 The Vertebrate Organ Systems 



the respiratory surface ; below it are numerous small blood vessels for 

 receiving the oxygen. Among some of the ocean-dwelling relatives 

 of the earthworm, folds of skin or simple gills develop. Gills are found 

 among many of the invertebrate groups, including the starfish, molluscs, 

 crustaceans, and some insect larvae. These gills are often elaborate 

 structures, much more complex than those of the marine annelids. 



Among the invertebrates, the insects and some of their allies have 

 developed one of the most efficient respiratory mechanisms. These forms 

 have fine tubes, tracheae, which penetrate throughout the body issues 

 and conduct the air to all parts. The efficiency of this system 

 is limited to comparatively small-sized animals ; these tracheal systems 

 are considered to be one of the important factors which limit the size 

 attainable by insects. 



The vertebrates show an even greater diversity of respiratory 

 structures. Among the fish and larval amphibians, elaborate gills are 

 developed in the pharyngeal region. Among the fishes the water enters 

 through the mouth and passes out through the gill slits. As the fish 

 swims about, a constant stream of water is passed over the thin mem- 

 branes of the gills, and oxygen is brought into close contact with the 

 small capillaries. 



With the advent of land life, the development of lungs and their 

 accessory structures appeared. The first lungs as seen among the 

 amphibians are simple sacs and provide but little surface for respira- 

 tory exchange. In this group, the moist skin is a necessary adjunct to 

 the lungs. Reptiles have more elaborate lungs with numerous infold- 

 ings of tissue for increasing surface. The best developed lungs are 

 found among the birds and mammals. This is undoubtedly associated 

 with the homoiothermic condition of these animals, which requires a 

 much more rapid oxidation of foods. The lungs of birds are connected 

 with air sacs which occupy the interior of bones and portions of the 

 coelom. These serve both to increase the efficiency of the lung ventila- 

 tion and to lighten the body for flight. Mammals have acces- 

 sory structures such as the diaphragm to increase the efficiency of air 

 movement in and out of the lungs. 



FUNCTION OF THE RESPIRATORY SYSTEM 



The respiratory system has the sole function of providing surfaces 

 for respiratory exchange. In so doing, oxygen for the production of 

 energy is made available for the animal, and the waste product, carbon 

 dioxide, plus some water, is eliminated. 



