192 r.] N. Annandale & R. B. S. Sewei.l : Vtvipara. 



289 



exactly the same type of excretory system. Infection with these 

 two cysts appears to occur in difierent stages of the life-history of 

 the mollusc host. Even in examples of so early a stage as xo mm. 

 in length, the gills have already become infected with the cysts of 

 this Xiphidiocercaria ; and out of 36 examples examined of sizes 

 ranging from 10 mm. to 18 mm. in length only two were apparent- 

 ly free from this parasite. With regard to the Echinostome 

 cysts in the auricle, however, infection appears to occur much 

 later, and further the proportion of infection is extraordinarily 

 different in the two sexes in early life. 



Table. 2.— -Showing the percentage infection with Echinioslome cysts 

 in the two sexes oj examples of 1920 brood. 



The table shows that infection is twice as frequent in young 

 males as it is in j'oung females. No case of infection was found in 

 examples that measured less than 14 mm. in shell length. 1 have 

 already mentioned that sexual maturity is attained in this species 

 when the individuals reach approximately the length of 13 mm. in 

 the & and 16 mm. in the 9 . Manson Bahr and Fairley (1920, p. 

 66) have stated, and my own observations on the cercariae of this 

 country have corroborated their statement, that " snails do not 

 become infested with cercariae till they have reached maturit3^ 

 that is about the third month." I have elsewhere put forward 

 the view that infection Ijj^ miracidia is probably largely dependent 

 on the establishment of a chemotactic stimulus at the time when 

 sexual maturity is attained in the mollusc host, and it seems 

 possible that we are dealing here with a similar phenomenon. 

 Certainly such an explanation would account for the higher per- 

 centage of infection in the males, which become sexually mature 

 at an earlier stage of their life-history than do the females, and 

 would also account for the freedom of infection of young imma- 

 ture examples. On the other hand infection of these molluscs by 

 the Xiphidiocercaria and the production of cysts in the gill-fila- 

 ments shows no evidence of any such phenomenon. 



Turning now to the presence of these cysts and the degree of 

 infection in adult individuals of either sex, I have given in the 

 table below the results obtained from a careful examination of 

 fifty examples, 25 aged i year and 25 aged 2 years. The point 

 to which I wish to call attention is the very large percentage 

 of 0* examples that show a heavy infection with both Xiphidio- 



