180 THE PHYSIOLOGY OF EARTHWORMS 



Shiraishi (1954). This author showed that carbon dioxide gas can 

 be detected by earthworms, if it is present in sufficient concentra- 

 tion. Animals moving along a tube can be induced to retrace their 

 path when met headlong by a stream of carbon dioxide gas. This 

 may offer an explanation of why earthworms leave their burrows 

 after rain. If the open ends of the burrows are sealed by water 

 during heavy rain a high concentration of carbon dioxide may 

 build up, and at the same time the oxygen content of the air may fall. 

 Carbon dioxide does not affect the respiration of earthworms 

 (Johnson, 1942) but oxygen lack is reflected by a need for greater 

 respiratory activity. But if the oxygen lack is only short-lasting 

 earthworms are able to tolerate it by undergoing a period of 

 anaerobic metabolism, repaying an oxygen debt later (Davis and 

 Slater, 1928). Shiraishi (1954) suggests that the gaseous carbon 

 dioxide can force the animals to leave the burrows, but this would 

 need a very high concentration of CO2 (see Chapter VII). This gas 

 will dissolve in water, however, to form a weakly acid solution of 

 carbonic acid, and as seen above acid sensitivity is a property of the 

 skin sense organs of earthworms, and it may be this stimulus that 

 causes vacation of burrows by earthworms, but this seems unlikely 

 in species living in base-rich soils. 



The sensory nervous foundation for these reactions are discussed 

 in the section on the nervous system. 



Learjiing and Habituation 



Thorpe (1956) has recently reviewed very fully the processes and 

 implications of learning and habituation throughout the animal 

 kingdom. Such processes are shown by even the lowliest of animals, 

 and the earthworm is no exception. 



Among the earliest experiments designed to investigate the 

 ability of earthworms to react to repeated stimuli and the con- 

 sequent learned series of actions, were the well-known studies of 

 Yerkes (1912 a, b). Specimens of E. foetida were put in the long 

 arm of a T-shaped maze and allowed a choice in the cross arm of, 

 to one side, a dark moist chamber in which the animal could 

 burrow, and in the other, an area lined with sandpaper, beyond 

 which the earthworm came into contact either with salt solution or 

 an electric shock or both. Individual animals showed considerable 

 daily fluctuation in the reaction to this problem, but it was usually 



