276 EllNKST WAKREN. 



isostonium it is not possible to prove that it enters into the 

 life-history of D. cirrigerum. In the particular crayfish 

 under consideration D. isostonium did not occur with any 

 absolute certainty, and according to Zaddach's theory the 

 eggs of D. cirrigerum should develop into the sexless D. 

 isostonium; but I have traced the development of the 

 sexual form direct from the egg in the crayfish host. 

 Hence we are led to one of two conclusions : either the eggs 

 of D. cirrigerum in the crayfish may develop direct into 

 the sexual D, cirrigerum or into the sexless D. isostonium, 

 according to the season and the particular physiological 

 condition of the host and parasite at the time in question; or 

 D. isostomum has no connection with D. cirrigerum, and 

 the development of the latter might be compared with that 

 of Aspidog aster conchicola, which inhabits the peri- 

 cardium or Keber's organ of Anodonta and other Lamelli- 

 branchs. 



Braun^ suggests that possibly the occurrence of the sexual 

 form in the crayfish is of the nature of an accident, and that 

 its typical and normal host is some vertebrate. 



Assuming that D. cirrigerum and isostonium are phases 

 in the life-history of the same species, it is probable that the 

 stimulus arising from change of host is necessary for the 

 production of sexual organs in D. isostomum, and the only 

 modification which is introduced into Zaddach's hypothesis is 

 the probability that the eggs of D. cirrigerum in the cray- 

 fish may according to circumstances develop into either the 

 sexual form which can complete its development in the same 

 host, or into a sexless form which must be transferred to some 

 other host before becoming sexual. 



If such be the life-history of the species it is unique among 

 trematodes; but the problem requires re-investigation in the 

 natural surroundings of the crayfish, and my own material 

 can throw no further light on the subject. 



' Bronn's ' Tliierreichs : Vermes, Trematodes,' Bd. iv, p. S70. 



