142 TJx'AyS/TlOX TO .1A.I/;A'0/?/0N/aS' 



a liiiili i)ercent;ige of water), the metabolic level will be 

 low and diffusion will suffice to distribute oxygen to all 

 living cells even at relatively low tensions. In still other 

 animals there is an efficient circulatory system, often 

 coupled with the presence of respiratory pigments (as in 

 cephalopods, crustaceans, and some worms), or the air is 

 brought in more or less direct contact with the cells by 

 means of tracheae (insects). 



The second group includes the following: Protozoa: 

 Spirostomum (Specht, 1935); Sponges (Moore, Edie, 

 Whitley and Dakin, 1912; Eaffy, 1933); Coelenterates : 

 the actinians (Trendelenburg, 1909; Henze, 1910; Har- 

 nisch, 1932; Eaffy, 1933), Cassiopea (McClendon, 1917) 

 and, to some extent, the corals (Yonge, Yonge and Nich- 

 olls, 1932; Kawaguti, 1937); Worms: many parasitic 

 nematodes, trematodes and cestodes (Harnisch, 1932a 

 and 1933a ; Friedheim and Baer, 1933 ; Kriiger, 1936 ; La- 

 ser, 1944), nemerteans (Raffy, 1933), some leeches (Raffy, 

 1933), Sipunculus (Heuze, 1910; Raffy, 1933) and, to a 

 lesser degree, Urechis (Hall^ 1931) ; Echinoderms: adult 

 starfish, sea-urchins and some holothurians (Hyman, 

 1929; Nomura, 1927; Meyer, 1935; Maloeuf, 1937) and 

 gastropods ( Thunberg, 1905 ; Dahr, 1927 ; Harnisch, 1932 ; 

 Fischer, 1931); Arthropods: Liniuhis (Amberson, May- 

 erson and Scott, 1924; Maloeuf, 1937a) and some crusta- 

 ceans (Moore, Edie, Whitley and Dakin, 1912; Amberson, 

 Mayerson and Scott, 1924; Chen, 1932). 



An explanation for the failure of some of these organ- 

 isms to secure a maximal amount of oxygen even at high 

 tensions was first attempted for the actinians by Henze 

 (1910) and Krogh (1916). According to these investi- 

 gators, the oxygen demand of the external layers of the 

 body of these relatively large animals is so high that 

 most, if not all the oxygen that enters the surface by dif- 

 fusion is consumed before it reaches the inner parts. 

 These latter receive either an insufficient amount or none 



