RESPIRATION 73 



as necessary for the purpose. In the absence of carbon dioxide 

 the blood of such fresh-water genera as the eel, pike, and carp 

 is half saturated at a partial oxygen pressure of 2-3 mm. (15° C). 

 In the marine species, represented by the cod and plaice, and 

 also in the trout, 50 per cent, saturation at 15° C. requires 

 an oxygen- tension of considerably greater dimensions, viz. 

 18 mm. with the cod, and about 11 mm. with the plaice or 

 trout. In both cases the presence of CO2 greatly diminished 

 the oxygen affinity of the blood at low tensions. In fresh water 

 the oxygen content is very variable and may sink to extremely 

 low values. The low loading tension of the blood in the fresh- 

 water fishes is thus appropriate to their medium, and accounts 

 for the low oxygen pressure to which Leuciscus (the minnow) 

 can, according to Winterstein's data, be subjected without 

 harm. In sea- water the dissolved oxygen is practically always 

 present in abundance at all depths. The water of the sea is 

 practically saturated with oxygen ; it therefore has an oxygen 

 tension above the 70 nmi. which represents the tension below 

 which (at 15° C.) a cod suffers from oxygen- want. In associa- 

 tion with this is the fact that sea- water fishes are very sensitive 

 to oxygen- want. The oxygen dissociation curve explains the 

 sensitiveness which makes sea-water aquaria more difficult 

 to maintain than fresh-water. The trout, which, as we have 

 seen, has a higher oxygen loading tension than such typical 

 fresh- water fish as the pike and carp, will only live in well- 

 aerated water, and is easily killed when the water is not 

 renewed or is insufficiently aerated. 



Barcroft and Barcroft investigated not only the entire 

 range of the dissociation curve for haemoglobin in Arenicola, 

 but compared its chemical and physical properties with that 

 of human haemoglobin. As regards the first, which are 

 graphically set out in Fig. 17, it is to be noted that the blood 

 of Arenicola has at low tensions a very much higher affinity for 

 oxygen than mammalian blood. The oxygen loading tension 

 is of the same order as that described by Krogh and Leitch in 

 fresh- water fish. Complete saturation is obtained at an oxygen 

 tension of 10 mm. There are, apart from this, two charac- 

 teristic differences between the haemoglobins of Arenicola and 



