THE PHYSICAL ENVIRONMENT OF ORGANISMS 



549 



For aquatic organisms, the interchange is between the gases of the 

 tissues and the gases dissolved in the water. Since there is no evapora- 

 tion problem and since mechanical support by the water permits a com- 

 paratively large development of surface area, the utilization of the body 

 surface is much more practicable for aquatic than for terrestrial creatures. 

 But even among aquatic organisms there are a number of factors that 

 limit the usefulness of the body surface for respiration. Increasing size 



Fig. 32.5. A sedentary food strainer of shallow marine waters. A model of the hard-shell 

 clam, Venus mercenaria, with the gills of the exposed side cut away to show the remaining 

 structures. Beating of the cilia which cover the surfaces of the gills and mantle causes 

 plankton-bearing water to flow into the spaces around the body and between the gills. 

 The water enters pores in the gills and is carried by canals to the excurrent opening; the 

 food particles remain on the surfaces of the gills and are swept forward into the mouth. 

 (Courtesy American Museum of Natural History.) 



or metabolic rate may finally make the surface area of the body inade- 

 quate for a sufficient gaseous exchange; motile or burrowing habits may 

 require an integument too thick and tough to serve as a respiratory 

 membrane; and the necessity for streamlining limits the amount of 

 surface area that can be utilized for respiration. Under these conditions, 

 the aquatic organism has to provide some special respiratory device, 

 nearly always areas of thin tissue through which rapid gaseous interchange 

 can be carried on between the body fluids and the water. These aquatic 

 respiratory organs, or gills, are of many types. In fast-swimming or 

 bottom-dwelling forms gills are usually enclosed in some protected 

 chamber and aerated by special breathing movements. 



