NEUROSECRETION 131 



migrations, and development of secondary sex characters. Oligo- 

 chaetes exhibit activity cycles, diurnal respiratory cycles, annual 

 reproductive cycles and in some cases a periodic aestivation. Some 

 or all of these functions may be controlled by neurosecretory 

 methods though evidence is still wanting in many cases. 



For example Abeloos and Avel (1928) worked upon the regenera- 

 tion of the body of Allolobophora (two species). The anterior end of 

 the body can be replaced at all times of the year, but the posterior 

 extremity can be regenerated at certain periods. This latter function 

 is an annual cycle concurrent with diapause (aestivation) and they 

 postulated that it is probably regulated by some internal mecha- 

 nism. Bailey (1930) also found that the regenerative ability of E. 

 foetida exhibits an annual cycle. 



Michon (1949) has since shown that aestivation is controlled to a 

 large extent by the conditions of soil moisture and humidity. As the 

 water available to the animals is decreased so they are more 

 likely to go into a condition of diapause, but Michon was unable to 

 explain why it is not possible to reawaken the animals when water 

 is provided in abundance during the diapause period. Once the 

 earthworms are aestivating a certain time must elapse before they 

 again become active. This is suggestive that a hormone is involved 

 regulating the activities of the species by an annual secretory cycle. 

 Long before this Avel (1929) had shown that when the temperature 

 is maintained constant, humidity is kept at a satisfactory level, and 

 feeding is continued, earthworms will go into diapause anyway. 

 This occurs at the end of spring when the reproductive period is 

 over and is accompanied by a loss of sexual characteristics such as 

 regression of the clitellum. 



Even earthworms that do not undergo aestivation e.g. L. 

 terrestris^ exhibit a yearly reproductive cycle, the gonads coming to 

 maturity in spring and early summer and regressing later in the 

 year to redevelop in the following spring. Of the types of secretory 

 cells in the cerebral ganglia one, the <2-cell, is apparently involved in 

 this periodic change, but the ^-cell is not. Individuals in which the 

 reproductive system has not yet matured are lacking in <2-cells, but 

 they differentiate as the animal ages. In adult animals exhibiting 

 the yearly maturation of sex organs the «-cells discharge their 

 contents in spring and the number of cells decreases. If the gonads 

 are extirpated the colloid content of the rt-cells decreases as the 



