RESPIRATION 65 



the lungs and gills with but little diminution of total respira- 

 tory activity. The following data from Krogh's (1904) 

 experiments show that the skin is a very important factor 

 in the respiration of the frog, especially when it is pointed 

 out that the total surface of the skin is only about one and 

 a half times the internal surface of the lungs. 



Where a respiratory pigment is present it is sometimes 

 possible to recognise its respiratory function by the colour 

 of the blood which enters and leaves an organ. This is true 

 of course of the lungs and gills of vertebrates, from which the 

 blood issuing is a brighter red owing to the formation of 

 oxyhaemoglobin to be discussed below. Analogous evidence 

 points to the conclusion that the gills of the cephalopod — 

 whose arterial blood is of a more bluish complexion than that 

 in the veins — are to be regarded as structures specialised for 

 the intake of oxygen from the surrounding medium. This 

 is confirmed by direct measurements of Winterstein (1908) 

 on the oxygen and carbon dioxide content of arterial and 

 venous blood in Octopus vulgaris. 



Arterial and Venous Blood in Living Octopus 



It is highly probable, but not proven, that the gills of 

 Crustacea are to be regarded in the same light. But it is not 

 wise to assume that all structures labelled gills by systematists 

 are special arrangements to facilitate respiration. This is 

 well illustrated by experiments of Fox (1920) on Chironomus. 

 This author employed an ingenious method to investigate the 

 localisation of respiratory exchange in minute organisms, by 



F 



