84 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Ten weeks (November to February) after a 

 small drill was exposed to about 50 miracidia, 

 serial sections revealed 15 well-developed rediae 

 in the head, foot, visceral mass, in the aorta near 

 the heart, and, for the first time in this study, 

 in the anterior margin of the digestive gland, 

 favorite site in heavy natural infections. Most 

 of the rediae contained daughter rediae which, in 

 turn, contained germ balls of a third generation 

 (fig. 6). This degree of development is probably 



Figure 6. — Bedia with daughter rediae, one of which 

 contains genn balls of a third generation, in foot ; 

 10-week experimental infection ; G, DH-AzB-E ; X230. 



attained sooner at the higher water temperatures 

 prevailing during spring and summer. 



The entrance of i-ediae into the digestive gland 

 marks the beginning of a concentration of para- 

 sites there, as is found in natural infections. 



Rees (1940) notes tliat the parent (first gen- 

 eration) redia gives rise to 20 or more daughter 

 rediae and that daughter rediae produce only cer- 

 cariae. If daughter rediae produce only cer- 

 cariae, it would appear that the parasites would 

 complete their development in a relatively short 

 time and depart, leaving the snail host infection- 

 free. However, the present study has demon- 

 strated that natural infections persist for at least 

 2 years. 



Sections of snails harboring natural infections 

 known to be at least 2 years old always iiad large 



numbers of daughter rediae containing cercariae 

 in various stages of development. Although no 

 daughter rediae containing recognizable rediae 

 were found, there were occasional small rediae 

 containing embryos which might have developed 

 into either rediae or cercariae. The evidence sug- 

 gests that there occurs a series of redial genera- 

 tions before final production of cei'cariae. 



Observing the prominent procuscula (posterior 

 "feet") of young rediae and their considerable 

 activity when removed from the snail, Rees (1940) 

 concluded that the rediae are capable of migrat- 

 ing among the follicles of the digestive gland. 

 The present study demonstrates rediae in various 

 stages of development in a wide variety of lodg- 

 ment sites, the location of which depends on how 

 long the snails have been experimentally infected. 

 In addition, rediae whose intestines contained yolk 

 platelets have been found in the digestive gland 

 of a naturally infected snail (fig. 9), indicating 

 that they had migrated from the ovary into the 

 digestive gland. It can be concluded, therefore, 

 that the redia is capable of migrating from an in- 

 vasion site anywhere on the body of the snail to 

 the final lodgment in the digestive gland or gonad. 



Although all experimental drills kept alive for 

 more than 3 days were maintained in running sea 

 water with adequate food to keep them in good 

 physiological condition, it is highly improbable 

 that any could have become infected by miracidia 

 brought in by the incoming sea water; because, 

 although simultaneously supplied by the same salt 

 water tap, none of the control snails of these ex- 

 periments, and none of 27 control drills of a lon- 

 gevity study of Parorchis-iniected drills similarly 

 maintained for more than 2 years, ever became 

 infected by Parorchis or any other trematode. 



Cercarial Encystment. — The swimming and 

 creeping motion of the cercaria was described by 

 Stunkard and Shaw (1931), who noted that it 

 attached readily to any surface and encysted soon 

 after attachment. In the present study, however, 

 the cercariae usually alternated between attach- 

 ment and creeping or swimming many times be- 

 fore finally encysting. 



Accelerated encystment following either me- 

 chanical stimulation, such as stirring or shaking, 

 or chemical stimulation, such as use of too con- 

 centrated solutions of vital dyes, was noted by 



