STRUCTURE AND FUNCTION IN INVERTEBRATES 



but certainly not homologous with the respiratory organs of other echino- 

 derms. Only rarely do terrestrial animals, possess blood gills, and those 

 that do, like terrestrial isopod crustaceans, are strictly limited to very 

 moist environments. 



Adaptation to the typically dry conditions of life on land has involved 

 the development of either lungs of various kinds or of tracheal systems. 

 The lungs of pulmonate gastropod moUusks are modifications of the mantle 

 cavity and its lining; and the "book lungs" of spiders suggest the plate- 

 like gills of some aquatic chelicerates, withdrawn within a protected cavity. 

 The tracheal systems of terrestrial arthropods such as insects consist of air 

 tubes through which atmospheric air is brought into the body. It may be 

 recalled that readaptation to an aquatic environment has occurred among 

 both pulmonate gastropods and tracheate arthropods, but never has there 

 been a return to the blood-gill system. Adult forms continue to use at- 

 mospheric air, but aquatic immature stages of insects often develop tracheal, 

 gills. 



The functional relationships are similar in all these adaptations; a thin 

 layer of cells separates the blood or body fluid of the animal from the external 

 water or air that is the source of oxygen for the animal. The site at which 

 oxygen is gained by the blood is also the site at which carbon dioxide is 

 eliminated from the body, whether it be over the general body surface, at 

 gills, in lungs, or in tracheae. Thus it is clear that there is no discontinuity 

 in these functions betwen invertebrates and vertebrates, which also possess 

 gills or lungs. As previously indicated the processes of cellular metabolism are 

 also fundamentallv similar throughout the animal kingdom; oxygen is utilized 

 within the cells in enzymatically controlled, sequential reactions involved in 

 the transformations of energy which are the basis of all metabolism (see 

 pp. 35 38). There are special cases of animals, notably intestinal para- 

 sites, inhabiting environments in which free oxygen is unavailable or present 

 in very low concentration. The metabolism of these forms chiefly involves 

 only the anaerobic phase. 



External gills Body surface Body surface External gills 



'^ ■ Tracheal system 



Mouth 



Lung Spiracles Respiratory tree 



("water lung") 



Fig. 17.4. Schematic dia£;ram representing a composite of many mechanisms of gaseous ex- 

 change found among various invertebrate animals. 



517 



