LUNGS 



323 



of the gasteropod mollusks, however, the ctenidium is not present, 

 and another organ, an invaginated "lung" is formed in the mantle 

 cavity to use air as a respiratory 

 medium. 



The inner surface of this lung 

 is lined with the respiratory cells, 

 which must be extraordinarily 

 efficient if their structure can be 

 taken as a criterion of their abil- 

 ity as a medium of gas exchange 

 (Fig. 289). They are so flat and 

 thin that they can with difficulty 

 be distinguished, in section, from ^ 

 the cells they rest upon. The 1 

 nucleus is of fair size, and is sev- 

 eral times the general thickness of 

 the cell body. At the point where F I G - 

 it lies, the cytoplasm is thickened 

 to contain it. 



In the lung tissue of a wood snail, Triodopsis tridentata, this flat 

 epithelium covers the greater part of the cavity, especially such parts 

 as are provided with the underlying blood channels. The cells bear no 

 cilia, but in some parts of the cavity are portions of the epithelium that 

 are composed of thicker cells, and these cells are provided with cilia. 

 Such organs of motion are necessary in a cavity that is constantly in- 

 vaded by particles of foreign substances carried with the air supply. 

 They are so arranged that their concerted action passes the foreign 



5. A surface view of some of the epi- 

 thelium that lines the mammalian lung. Ni- 

 trate of silver preparation. (From Stohr's 

 " Histology," after Lewis.) 



FlG. 289. Vertical section of a part of the combined body wall and lung wall of a wood snail, 

 Triodopsis. s.ep., external shell epithelium; mus.f., two layers (longitudinal and circular) 

 of muscle fibers; bl.ca., layer of blood capillaries, one of which contains a blood corpuscle; 

 c.w., single-layered capillary wall; res.e., respiratory epithelium. It clings to the outer sur- 

 faces of the capillary walls in nature and is shown artificially separated in the left part of 

 the figure. X 730. 



particles along in the thin covering of mucus that covers them and 

 expels the whole mass from the pulmonary opening. 



The relations of the respiratory cells to the blood vessels on which 



