98 THE PHYSIOLOGY OF EARTHWORMS 



of the body wall also help to move the blood through the vessels 

 but pressure within the vessels is always very low, ranging from 

 4-4— 5-5 mm Hg when the animal Lumbricus is at rest up to some- 

 thing more than 9-3 mm Hg when active (Prosser et al.^ 1950). 

 Extensive blood capillaries are present in both body and gut 

 walls. 



The fluid circulating within the vessels is red in colour due to 

 the presence of the respiratory protein haemoglobin. This sub- 

 stance is not contained within corpuscles as in vertebrates but is in 

 physical solution in the plasma. Amoebocytes are present in the 

 blood which carry out a phagocytic action, but are unable to 

 traverse the walls of blood vessels (Tuzet and Attisso, 1955). 

 These cells are not associated with haemoglobin. Not much is 

 known of other blood constituents such as the ions, sugar, amino- 

 acids, etc. 



Haemoglobin 



The function of respiratory pigments is to combine with 

 oxygen at the surfaces available for such a reaction i.e. the body 

 wall of oligochaetes and transfer it to the various tissues where it is 

 released as required in response to a low oxygen tension. It is 

 possible to envisage three methods by which such an action may be 

 carried out. 



(1) Transport and release of oxygen by haemoglobin may occur 

 incessantly during life. 



(2) The haemoglobin may only come into play when oxygen 

 tensions outside the body fall to such a low level that simple 

 diffusion inwards through the surface is no longer sufficient to 

 keep a high level of dissolved oxygen in solution in the blood 

 plasma circulating round the body, or 



(3) The respiratory pigment may act as a temporary store of 

 oxygen to be used only in times of severe oxygen stress (Fox, 

 1940). The appropriate position of earthworm haemoglobin in the 

 above scheme of things was unsettled until the work of Johnson 

 (1942). The earlier experiments of Jordan and Schwarz (1920) and 

 Dolk and Van der Paauw (1929) suggested that haemoglobin in 

 Lumbricus acted only as a store to be used as and when oxygen 

 stress was encountered. Thomas (1935) criticized these conclu- 

 sions and on the basis of experiments with five worms decided that 



