104 A. r. THOMAS. 



the numerous conjectures already expressed, some of which 

 had very scanty evidence to support them. Work was begun 

 in two directions, first, by endeavouring to infect many of our 

 commoner molluscs, both terrestrial and fresh-water. In the 

 second place, numerous localities in the country around Oxford, 

 in which sheep were known to have incurred liver-rot, were 

 visited, the whole of the invertebrate fauna was carefully 

 studied, and many specimens brought home to be dissected 

 and searched for larval trematodes. Very numerous places were 

 examined in this way, but mention will only be made here of the 

 one which proves finally to have given the correct solution of 

 the problem. At Wytham, near Oxford, was a clearly circum- 

 scribed area of infection. The fields, five in number, were 

 situated on the side of a hill far above the reach of river 

 floods, lying upon the Oxford clay. They were searched 

 thoroughly by day and by night, and the various invertebrate 

 animals found, including snails, slugs, aquatic insect larvfe, 

 Crustacea, worms, &c., brought home and examined. Fresh- 

 water snails Mere very scarce on the ground ; all that were 

 brought to light were two small specimens of Physa fonti- 

 nalis, a small Cyclas, and Limn sens truncatulus in 

 moderate numbers. The last-named species was found in a 

 boggy spot in one of the fields. In one of them was discovered, 

 on the 22nd December, 1880, the peculiar and interesting form 

 of cercaria to which allusion has already been made. Its most 

 striking character was due to the presence of very coarsely 

 granular cells arranged in lobed masses along each side of the 

 body. It was very active, but soon came to rest, encysting 

 itself upon surrounding bodies. The cyst was snowy white, 

 from the presence in its substance of the highly refractive 

 granules already seen in the granular cells forming the lateral 

 masses, which were thus shown to be cystogenous organs. 



The other points in the structure of the cercaria were all 

 favorable to the supposition that 1 had here discovered the 

 long-sought cercaria of the liver-fluke. I had already found 

 in a lamb's liver exceedingly minute flukes, smaller than any 

 yet recorded, one of them being only Tl mm. in length, i.e. 



