84 



THE PHYSIOLOGY OF EARTHWORMS 



In a few cases, for example Dero and Branchtodrilus, there are 

 gills presenting a large surface area for the purposes of gas exchange 

 (Fig. 24). This situation contrasts greatly with the great profusion of 



Fig. 24. Branchiodrilus hortensis showing the distribution of the 

 gills (from Stephenson, 1930). 



gill Structures in polychaetes. In earthworms about which w^e have 

 most knowledge, the only structural specialization consists of 

 looped and branched capillary extensions of the blood vessels in the 



