The life history of Diplodiscus temporatus Stafford. 607 



show clearly the striking differences between the cercariae of Biplo- 

 discus temporatus and Amphistomum subclavatum. The shape of the 

 hody is very different. The comparative length and diameter of 

 the tail in the two species also differs very markedly. The most 

 fundamental difference, however, is found in the relation of the water 

 vascular system to the tail. In A. subclavatum the canal leading 

 from the excretory bladder extends nearly the entire length of the 

 tail, near the end of which it opens to the exterior through two 

 excretory pores, one on either side of the tail. In D. temporatus, as 

 previously described, the excretory pore is on the dorsal surface of 

 the body just anterior to the tail. Looss also figures the testes as 

 forming a single mass which extends across the body from the in- 

 ternal wall of one caecum to that of the other; while in D. tem- 

 poratus there are two distinct primordia, one for each of the testes. 



4. The swarming of the cercariae. 



Pagenstecher (1857), Looss and Lang have describe the 

 swarming and encystment of Amphistomum suhclavatum. These 

 authors agree that the cercariae are born (set free from the rediae) 

 in an early stage of development. For some time the young cer- 

 cariae wander about among the organs of the host until they have 

 completed their development, when they force their way to the 

 surface and become free in the water. According to Lang they 

 may escape through the digestive tract of the host; Looss maintains, 

 on the contrary, that they reach the exterior through the respi- 

 ratory cavity of the snail. 



In all the specimens of infected snails that I have examined I 

 have never found a single individual in which there were any young 

 cercariae free in the body of the host. In the comparatively few 

 instances where free cercariae were found they were always fully 

 developed and ready to encyst. The only snails in which free cer- 

 cariae were found were some from which mature cercariae had al- 

 ready been seen to escape. 



In the observed instances where mature cercariae were escaping 

 from the snails the period of liberation extended over about sixty 

 hours. In every instance it appeared that all of the cercariae in 

 any snail were of practically the same age, so that after a period 

 of swarming no cercariae would be set free until a new "brood" 

 had matured. The sectioned material, whatever was the stage in 

 the development of the cercariae, showed that in any snail all of 



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