104 RESPIRATORY MECHANISMS 



monoxide (7 mm) which saturates the blood, but does not 

 otherwise interfere with the metabolism or activity. 



The fresh-water snail Planorbis corneus, which frequents 

 stagnant pools and often stays for hours at the bottom, has a 

 blood volume amounting to 1/2 of the weight, containing Hb 

 with an oxygen capacity of 1 to 1.5 vols. %. The snail comes 

 to the surface occasionally and takes air into the lung, but it 

 can also utilize dissolved 2 through the skin and a functional 

 "gill" (Leitch, 1916; Borden, 1931; Wolvekamp, 1932). 

 The inactivation of the Hb made no difference down to ten- 

 sions of about 50 mm, but at lower tensions the Hb came 

 definitely into use. Its t u is at 20° and without C0 2 about 

 6 mm. 



The common earthworm (Lumbricus) was studied by similar 

 methods by Dolk and van der Paauv (1929) with very similar 

 results. The haemoglobin does not come into use until the 2 

 pressure is reduced to 55 mm and can maintain an adequate 

 supply down to less than 20 mm. Even at 3 mm 50% of the 

 normal metabolism can be obtained. 



The holothurian Caudina and the bivalve Anadara (Area) 

 have haemoglobin containing corpuscles which are utilized 

 at rather low tensions, 8-10 mm, according to Kawamoto 

 (1928). 



Isabella Leitch (1916) also studied the blood of red Chiro- 

 nomus larvae which have the steepest dissociation curve so far 

 observed. At 20° the t u is about 0.2 mm and there is a 

 definite C0 2 effect reducing the saturation at 0.2 mm 2 and 

 8 mm C0 2 to 39%. These larvae live like the Tubifex worms 

 mentioned on p. 30 in vertical tubes built from the muddy 

 bottom of lakes and ponds where the oxygen tension is nor- 

 mally low and may become reduced to 0. It can be observed 

 directly when the transparent larvae are viewed through a 

 microspectroscope that the haemoglobin does not begin 

 to function until the 2 tension in the surrounding water is 

 below 7 mm. Pause (1919) found that red larvae of Chiro- 

 nomus gregarius could live in water with 0.2 ml 2 /l (corre- 

 sponding to a tension of 4-5 mm), but were killed in a few 



