1920] Johnson: Life Cycle of Echinostoma Revolutum (Froelich) 371 



although the rediae were abundant. The temperature of the water is 

 probably the controlling factor. The temperature of water of three 

 lakes about the size of Stow Lake has been kept daily for several years. 

 These three lakes are close to Stow Lake and are about the same size, 

 so that their temperature probably differs but little. During the three 

 winter months the average temperature of the water is 52° P, occasion- 

 ally dropping as low as 48°. Beginning the first of March there is a 

 quite rapid and steady rise in the temperature, which is accompanied 

 by a corresponding increase in the number of active cercariae. During 

 the three warmest months, July, August, and September, the tem- 

 perature averages 67° F, occasionally reaching 70°. During these three 

 months and the month following the active cercariae continually 

 increase in number. It seems obvious that the degree of temperature 

 controls the development of cercariae, the mother rediae and the 

 enclosed daughter-rediae. 



There are four ways by which stages other than the adult of 

 Echinostoma revolution can pass through the so-called winter season 

 in the bay region of California. The first way is by means of the egg. 

 The second way is by means of the mother-redia. This could only 

 result from eggs being produced during the late summer or early fall. 

 During the process of metamorphosis from the miracidium into the 

 mother-redia, or after the enclosed daughter-redia had grown nearly 

 to full size, the coming of winter would force the mother-redia to 

 remain dormant until spring. The third way is by means of the 

 daughter-rediae, i.e., rediae containing cercariae. This could result 

 only from eggs produced during mid-summer. There would be suffi- 

 cient time for the mother-redia stage and nearly enough time for the 

 daughter-rediae to produce active cercariae. The lack of time to 

 mature the cercariae would force the daughter-rediae to remain dor- 

 mant until spring. The fourth way is by means of the encysted 

 agamodistomes. In a climate where there is a definite winter season, 

 I doubt if the second and third ways of surviving the winter exist. 



The length of life of echinostome rediae has never been determined, 

 but doubtless they exist for a year or more. The reason for this con- 

 clusion is. that since the average number of active cercariae in the 

 rediae is five, and the total number of germ balls of all sizes is over a 

 hundred, probably closer to two hundred, it seems impossible that all 

 the germ balls could grow to maturity in less than a year's time. That 

 they exist in a dormant or semi-dormant condition for the four months 

 of the winter season is known. 



