RESPIRATION IN WATER 



27 



oxygen is absorbed (CO- 2 eliminated) than corresponding to 

 the metabolism of the organ itself. 



The surface of an animal can be enlarged, either by being 

 turned in to form cavities of increasing complexity, or by being 

 turned out to form appendages. We shall designate all cavi- 

 ties serving respiratory purposes as lungs and all respiratory 

 appendages as gills. As will be shown in some detail below, 

 lungs are the typical organs for air breathing and gills for 

 the respiration of dissolved oxygen. Nevertheless we find a 

 few examples of lungs for water breathing and a larger 

 number of gills for air breathing. Both lungs and gills often 

 serve other purposes besides respiration, and it is sometimes 

 difficult to decide whether cavities or appendages have any 

 definite respiratory function. 



Respiratory organs have been described in several minute 

 worms possessing a well-developed circulatory system (e.g., 

 Dero). Their function as such is extremely doubtful. 



Generally a convection transport of oxygen by a circulating 

 fluid is essential for the func- 

 tion of respiratory organs, 

 but there are certain excep- 

 tions, and we have in the 

 tracheal respiration, to be 

 dealt with in a separate 

 chapter, a system which is 

 independent in principle of 

 any circulation. 



Water lungs are found in 

 the Holothuria (Fig. 7) . They 

 are branched thin -walled 

 tubes stretching from the 

 anus throughout the length 

 of the body. By contrac- 

 tions of the body wall they 

 are at irregular intervals 

 emptied (completely?) and 



Fig. 7. Water lungs (resp.) of a 

 Holothurian, opened along the middle of 

 the dorsal surface. (Leuckart.) 



