DISTOMUM HEPATICUM 15 



thenogenetic), and the sign - to denote the simple growth of 

 a larva into a mature form, we may express the alternation of 

 generations of Distomum as follows : — 



^ Fluke 

 ' Miracidium 



Spoiocyst . 



II 

 Redia 



II 

 (Redife)n. . 



Sexual generation in sheep 



Free swimming larva 



1st type of asexual generation in snail 



2nd type of asexual generation in snail 



Successive generations of 2nd type 



Cercaria . . Voung sexual generation 



I 



^ Fluke . . . Mature sexual generation 



This complicated life-history is obviously connected with the 

 parasitic habit of the animal. Living in the bile ducts of-a 

 sheep, and eventually causing the sickness and death of its 

 host, it is necessary for the propagation of the species that 

 Distomum should be from time to time transferred to a new host, 

 and this transference from the liver of one herbivorous animal to 

 another is not easy of accomplishment. Hence we find that, in 

 the first place, the mature fluke produces an inordinate number 

 of eggs, some few of which are sure to develop under con- 

 ditions favourable to survival. The miracidium hatched out 

 from the egg is a larva structurally adapted to seek out and 

 enter an intermediate host, and it is interesting to observe that 

 this intermediate host, Limnaa truncatula, is precisely the one 

 whose habits ensure the widest possible dispersal of the asexual 

 generations. Although its habit may generally be described as 

 aquatic, L. truncatula is more ready to leave the water than 

 other species of water snails with which it is associated, and it 

 commonly makes considerable journeys through damp grass. 

 It may even be dried up, and recover its vitality when wetted. 

 During floods L. truncatula may be carried in great numbers 

 over low-lying lands, and when the waters have subsided 

 numerous specimens are to be found scattered over the 

 meadows or living in the ditches at their sides. Thus, when 

 the redias living in infected snails give rise to cercarise, the last 

 named frequently escape in situations where they are likely to 

 be picked up by sheep pasturing on the grass on which they are 

 encysted. It need hardly be said that the chances against the 



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