MARSHALL: ON A PARASITE OF LIMN^A TRUNCATULA. II 



is laid aside, and in about a fortnight the embryo becomes an 

 adult sporocyst. But in the sporocyst itself (to follow up this 

 strange eventful history) change soon occurs, and within is 

 visible a numerous progeny of redia: creatures resembling the 

 sporocysts or brood-cysts, only that they are provided with an 

 intestinal tract. These redice. continue to grow until the 

 sporocyst is nothing but a bag surrounding them — a bag that in 

 due course parts and sets them free, as they are successively 

 matured, within the pulmonary cavity of the snail, which I may 

 mention they soon exchange for the liver of the moUusk, But 

 just as the redice. were a progeny within the sporocysts, so within 

 the redice. themselves another progeny arises, sometimes of 

 daughter redm, but more frequently of quite another form, viz., 

 the tadpole-like cercaria, the last a long known ' form, though 

 only comparatively recently recognised in its due place and 

 relationship. The cercarice. at last escape by a special aperture, 

 and leave, first their nurse and then their snail host, and embark 

 on a roving career in the water. But this fitful life (every stage 

 of which has again and again been seen) draws to a close, and 

 the cercarice adheres at last to a blade of grass by the stream or- 

 pond side, casts off its tail, and encysts itself And there 

 matters for it will end, and it will enter upon a long, perhaps 

 an eternal sleep, unless its grass-blade should be swallowed by 

 another animal, and best of all by a sheep. Should this occur, 

 it will commence to live actively once more, and will finally 

 attain in about six weeks its summum of development as a 

 perfect liver-fluke. From this proceed the countless and tiny 

 ova with which our history began. A small bottle of this ova 

 is sufficient to kill every sheep in Great Britain ! We have, 

 therefore, these distinct and diverse forms — i, ovum ; 2, ciliated 

 embryo; 3, internally produced sporocyst; 4, internally produced 

 redia; 5, internally produced Cercaria; eventually passing into a 

 ■perfect J^ascioia hepatica — a series of transformations unparalleled 

 in any chapter of natural history. Such being the history, the 

 cure is plain. Keep sheep on dry pastures, provide them with 



