28 VERTEBRATE RESPIRATION 



one another. The size of the gill slits is increased and hence the 

 resistance to the reversed flow diminishes. In elasmobranchs 

 each branchial arch only has a single row of gill rays and it is 

 doubtful whether similar mechanisms are found. 



Further differences between fishes are found in the way they 

 vary the volume of water pumped across the respiratory epithe- 

 lium. Obviously this may be achieved by changing either the 

 frequency of the respiratory rhythm or the volume of water 

 pumped for each respiratory cycle (stroke volume). Relatively 

 few fishes have been investigated from this point of view. A trout 

 of 900 grams pumped 8 litres of water per hour at 10-12° C 

 when at rest. During more active ventilation, because of more 

 CO 2 or less O2 in the water, the respiratory volume may be 

 increased to 40 litres per hour. This increase is accomplished 

 mainly by a greater stroke volume, for the frequency remains 

 about 60 per minute at all ventilation volumes. In the eel, how- 

 ever, any increase in ventilation volume is achieved by a com- 

 bination of both stroke volume and an increase in frequency, 

 whereas in a tench it is achieved mainly by increasing the 

 stroke volume (fig. 9). 



