30 VERTEBRATE RESPIRATION 



(a) ACCESSORY RESPIRATORY ORGANS 



There are, however, quite a number of fish which are able to 

 breathe air and habitually do so because of the low Og content 

 of the water which they inhabit. These are usually found in 

 swamps; those of Paraguay and East Africa have been most 

 studied, where the concentration of oxygen in the water ranges 

 from 4-5-50 ccs per litre in the creeks, down to 0-2-2-0 ccs in 

 the swamps themselves. These low oxygen concentrations are 

 due to the stagnant conditions which allow the accumulation of 

 decayed products and in many cases the poor light which reduces 

 photosynthesis of any aquatic plants. Furthermore, the tempera- 

 ture of these waters is high and so is the CO 2 content; by no 

 means ideal conditions for aquatic life. Many different species 

 offish live in waters of this type, however, and it has been found 

 that they obtain their oxygen in one of two ways. The first group 

 obtains sufficient oxygen from the thin film at the surface which 

 is in contact with the air and remains relatively well oxygenated. 

 The majority offish live in deeper waters and possess some sort 

 of accessory breathing organs. These organs are found in many 

 parts of the body, usually a highly vascularised portion of the 

 alimentary canal that is covered with a thin respiratory epithe- 

 lium where gaseous exchange takes place. The fish comes to the 

 surface periodically to swallow air which passes to the appro- 

 priate part of the gut and oxygen is absorbed. The frequency of 

 the visits to the surface increases at lower oxygen tensions. 

 Such accessory organs are found in species from many diff"erent 

 groups of fishes and there is little doubt that they have been 

 independently evolved to meet the stringent requirements of 

 these habitats. Many striking examples of convergent evolution 

 are known. Correlated with the independent origin of these 

 organs, we find associated differences in their blood supplies. 

 They are usually supplied with blood from the same vessel that 

 would supply them if they were parts of a normal alimentary 

 canal (Table 4). Typically, for example, those which are modified 

 portions of the stomach, intestine or rectum receive their blood 

 from the coeliaco-mesenteric, or posterior mesenteric arteries, 



