86 



THE PHYSIOLOGY OF EARTHWORMS 



In all respiratory systems oxygen first dissolves in a watery layer 

 covering the respiratory surface. It proceeds from there into the 

 body by a passive diffusion, not by an active process. This has 

 been confirmed for earthworms by analysis of the gaseous exchange 

 in relation to body area and weight (Kriiger, 1952). The surface of 



Dorsal b.v. 



Lateral folds 



Anus 



Fig. 26. Exposure to the air of the posterior dorsal surface and 

 formation of "lung" in Alma emini. A. extrusion of hind end. B. 

 hollowing of dorsal surface. C.-E. dorsal view of extruded hind end. 

 Stages in the formation and closure of lateral fold and retreat into 

 mud. The dorsal blood vessel and numerous lateral connections 

 are clearly visible in this region. F. final position after retreat into 

 mud with tubular lung open to the air (from Beadle, 1957). 



the body is kept moist by the continuous secretion of mucus which 

 also serves a purpose as a lubricant during locomotion, and 

 cementing soil particles in the burrow. Desiccation leads to 

 increased mucus secretion, and in the last event to an expulsion of 

 fluid from the coelom onto the body surface via the dorsal pores 

 (Wolf, 1940). 



