142 



W. A. CASTLE. 



once to be temperature. During the months of September and 

 October when the small worms are emerging from cysts there is 

 a gradual fall in the temperature of the water. The maximum 

 reached during the summer is about 12-13 C. By the end of 

 October this has fallen to 8-9. The minimum is reached by the 

 end of December, and at that time completely differentiated sexual 

 worms have been collected from beneath the ice where the water 

 is no warmer than 5. During late January and early February 

 there is a pronounced decrease in the number of sexual worms be- 

 fore there is any appreciable rise in the temperature of the water. 

 During the spring there is a rise in water temperature accompanied 

 by the total disappearance of sexual structures. The majority of 

 the worms fragment and encyst during these months of rising 

 temperature ; some however persist through the summer as asexual 

 worms. 



Experiments have been carried out to determine the extent to 

 which sexuality is influenced by changes in temperature. First 

 it is noted that when sexual worms are brought from their natural 

 habitat and placed in the laboratory at a temperature of 18-20 

 there is a quite rapid disappearance of sexual structures. This 

 occurs irrespective of food conditions. By the end of two weeks 

 ovaries, testes, and atrial organs have completely vanished. This 

 is followed at once by the fragmentation and encystment of all the 

 larger worms. Medium sized and small worms become apparently 

 normal asexuals, but if fed will grow and undergo the usual pre- 

 cystic changes as soon as they have attained a sufficient size. 



The history of the majority of sexual stocks placed at a tem- 

 perature of 8-10 is not different from this, although if the ani- 

 mals are not fed very few of them will encyst unless they are ex- 

 ceptionally large. The disappearance of sexual structures is much 

 retarded testes and a genital pore may remain evident for from 

 four to six weeks after the date of collection. With the possible 

 exception of one lot of worms further sexual differentiation has 

 never been observed after the time of collection. In this single 

 case worms which were exceptionally large and well differentiated 

 sexually were isolated at 8 at the time of collection. Three eggs 

 were laid by these worms within twelve days of the time of isola- 

 tion. Subsequently to this the worms lost all sexual structures 



