108 RESPIRATORY MECHANISMS 



functions as an oxygen store, although the authors do not 

 appear to entertain this possibility (Prenant, 1900; v. Kem- 

 nitz, 1917). 



It is a very remarkable fact that quite similar organs occupy 

 one-fourth to one-half of the abdomen in one European 

 (Anisops) and one American genus (Buenoa) of Notonectida. 

 The authors describing these organs (Poisson, 1926; Bare, 

 1928) were unable to find any plausible function, but the cases 

 would probably repay a closer study from a physiological 

 point of view. 



In certain animals the storage of oxygen in the blood itself 

 appears to be of biological importance. The first example to 

 be studied in any detail (by J. Barcroft and H. Barcroft, 

 1924) was the worm Arenicola, burrowing in sand and mud 

 flats in the tidal zone, where during the ebb the access to 

 oxygen is very limited and there is even the danger that the 

 gas may diffuse away from the burrow into the surrounding 

 mud. This danger is minimized by the very steep dissocia- 

 tion curve of the blood. It was clearly shown in this case that 

 the total oxygen capacity of the very large blood volume would 

 be of definite assistance in tiding over the period of inadequate 

 supply, but not sufficient to cover the normal requirements. 



This was confirmed by Mabel Borden (1931) who showed 

 further that also Planorbis can draw to an oecologically sig- 

 nificant extent upon the oxygen stored in its haemoglobin. 

 Hazelhoff (Jordan, 1922) compared the O2 content in the lung 

 of Planorbis with that of the haemocyanin containing Limncea 

 when both were kept under water for three hours. In Limncea 

 the oxygen fell off in an hour to the low value of 6% and the 

 metabolism became much reduced. In Planorbis the 2 took 

 150 minutes to reach 4% and the metabolism was kept up, 

 thanks to the storage function of the haemoglobin (Fig. 63). 



A similar reasoning holds for the echiuroid worm Urechis 

 caupo, which works on the whole at somewhat higher oxygen 

 tensions, according to Redfield and Florkin (1931). Urechis 

 is exposed to a deficient oxygen supply during low tide, and 

 even when the flats are covered it may occasionally suspend 



