344 University of California Publications in Zoology [Vol. 19 



REDIAE CONTAINING CERCARIAE 



The rediae that produce the cercariae of Echinostoma revolution 

 are, I believe, those that have emerged directly from the mother-rediae, 

 which were probably derived by metamorphosis from miracidia. In 

 all structural characters the rediae producing daughter-rediae and the 

 rediae producing cercariae, seem to agree at maturity. 



The rediae are quite characteristic even though they vary greatly 

 in size, shape, color, activity and number of cercariae and germ balls 

 enclosed. 



Rediae containing active or mature cercariae vary from 0.31 to 

 2.80 mm. in length, the average being about 1.60 mm. The smaller 

 rediae contain few, usually only one. active cercariae and a small 

 number of germ balls. The larger ones may contain as many as eleven 

 active cercariae with perhaps seventy good sized germ balls, besides 

 many other smaller ones. Active cercariae are nearly mature and 

 always contain prominent concretions. The count of the active 

 cercariae was often made upon this characteristic, which was found to 

 be wholly reliable. Occasionally a large redia may contain only one 

 or two active cercariae and a few germ balls, and on a very few 

 occasions only two or three germ balls and no active cercariae. These 

 I judge to be old rediae producing their last cercariae before final 

 deterioration, as several entirely empty and apparently lifeless rediae 

 were seen. From fifty rediae taken at random the average number of 

 active cercariae proved to be five with also at least fifty fair sized germ 

 balls. 



This great variation in size of rediae is accounted for in two ways, 

 first, that there is no very definite adult size, and second, that mature 

 cercariae are produced long before the rediae are full grown. They 

 possibly continue to grow and to produce cercariae until the last of 

 the germ balls are matured. I am inclined to think that the first 

 explanation accounts for most of the variation. 



The great variation in number of germ balls contradicts Ssinitzin's 

 theory (1911) that the numbers of cercariae and germ balls of a given 

 species fall within a certain definite yet quite a large range. To a 

 certain extent the number of germ balls is determined by the size of 

 the redia, but there are plenty of exceptions. 



The shape of the redia is usually that of a stocking when viewed 

 from the side (figs. 26 and 27) and apparently straight, when viewed 



