Except for some larval forms, gills are ubiquitous in fish. The fine
structure of the gills of tcleosts has been described by Hughes and
Grimstone (1965), Rhodin (1964), Newstcad (1967), and Hughes and
Datta Munshi (1968 ). Other groups of fish have received less attention,
references and a general description of the anatomy of the gills of elas
mobranchs, cyclostomes, and Dipnoi can be found in Daniel (1922 ),
Fry (1957), Chapman et al. (1963), and Johansen and Strahan (1963 ).
The gills form a sievelike structure placed in the path of the respira
tory water Bow. The secondary lamellae form the side walls of this sieve
(Fig. 1) and probably represent the major respiratory portion of the
gill structure (Hughes, 1966a; Muir and Hughes, 1969 ). The total sur
face area of the secondary lamellae is about 5 cm' / g body weight (Gray,
1954; Hughes, 1966a ). There is a countercurrent (van Dam, 1938;
Hughes and Shelton, 1962 ) or multicapillary (Piiper and Schumann,
1967 ) arrangement of the Bows of blood and water on either side of the
gill epithelium; the epithelium is usually between 1 and 5 p. in thickness.
The ratio of the Bows of blood and water is somewhere between 1: 10
(Piiper and Schumann, 1967; Garey, 1967 ) and 1:80 (Stevens and Ran
dall, 1967b ). The ratios of the content per mm Hg partial pressure of
both oxygen and carbon dioxide in water and blood are between 1: 10
and 1: 20 (Black et al., 1966; Beaumont and Randall, 1968 ).
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