RESPIRATION 85 



body wall (Fig. 25). These contain haemoglobin and diffusion 

 accounts for the passage of gases to and fro. 



One species of oligochaete, however, is worthy of special 

 mention, namely Alma emini, a glossoscolecid oligochaete found in 

 the papyrus swamps of East Africa. It lives in a substratum of 

 decomposing vegetable matter that is strongly reducing and 

 probably devoid of free oxygen. Alma lives immersed in the mud 

 and very often only the rear end projects above the surface into the 

 air. This rear extremity is modified to form a highly vascularized 

 lung (Fig. 26). The edges of the body fold over to form a tube in 

 which bubbles of air are frequently trapped and taken down when the 

 animal retreats from the surface . The haemoglobin of this oligochaete 

 has a very low unloading tension, being oxygen saturated at a partial 



ep 



Fig. 25. Intra-epidermal capillaries oi Lumbricus . ep = epidermis; 



cm. = circular muscle; I.m. = longitudinal muscle (from 



Stephenson, 1930). 



pressure of 2 mm Hg oxygen in the absence of carbon dioxide. In 

 other words in the oxygen-deficient layer where the animal lives it 

 is able to make full use of the little oxygen which is available. In 

 an ecological habitat such as this carbon dioxide may be expected to 

 accumulate to high levels, and it is notable that high CO 2 tensions 

 are without noticeable effect on the dissociation curve of the 

 haemoglobin of A. emini. Only at a tension of 200 mm Hg of CO2 

 is the curve slightly displaced to the right, i.e. the unloading tension 

 increases (the Bohr effect) and worms can live for up to 48 hours in 

 water completely saturated with CO2. Although laboratory studies 

 with this species show that it can survive under completely anaero- 

 bic conditions its behaviour in mud: water culture suggests that 

 oxygen is a prerequisite for normal existence (Beadle, 1957). 



