342 University of California Publications in Zoology [Vol. 19 



miracidia metamorphose directly into rediae, which in this paper I 

 shall call mother-rediae. On several occasions very small mother- 

 rediae were obtained, measuring in length from 0.148 to 0.302 mm. 

 After thirty-five days in the tissues of the young snails some mother- 

 rediae had only increased 0.28 mm. beyond the length of the mira- 

 eidium, thus being in all only 0.148 mm. long in their most contracted 

 condition. The smallest contracted free daughter-redia ever seen 

 measured 0.180 mm. in length, or 0.032 mm. longer than mother-rediae 

 when five weeks old. If these young mother-rediae had been daughter- 

 rediae instead then there would also have been present large rediae, 

 which never were found even after the most careful examination. 

 Also if I had missed a generation, and these mother-rediae were really 

 daughter-rediae, then large numbers of daughter-rediae should have 

 been found. But never were more than three found in a snail and that 

 only once, the rest contained two or one mother-rediae or none. Also 

 the shape at this time is somewhat different from that of the daughter- 

 rediae. These four facts taken together make me feel certain that 

 there is no intervening stage between the miracidia and the rediae 

 found in the experimental snails. 



The variation in the size of these small mother-rediae is due par- 

 tially to the degree of contraction. They are more sensitive than 

 when mature since they contract and expand if slightly interfered 

 with on a slide. Figure 19 shows a mother-redia in four different 

 shapes due to the degree of contraction. Figure 19a shows the mother- 

 redia to be almost sac-shaped with no trace of the posterior protrusions. 

 Figures 196, e, and d show the posterior protrusions becoming more 

 prominent as the length increases. The blind intestine also shortens 

 and lengthens with the body. 



Because of the opaqueness of the body of the young mother-redia, 

 the germ balls could not be made out. How long it takes the mother- 

 redia to develop to maturity and to produce another parthenogenetic 

 generation is as yet undetermined, but probably it is accomplished in 

 about three months during the warmer weather. 



REDIAE CONTAINING DAUGHTER-REDIAE 



In thirteen snails of Physa occidentalis, taken from Stow Lake, 

 San Francisco, rediae with daughter-rediae inside, were found. 

 Although I have never obtained very small rediae containing daughter- 





