Experiments on the Development of the Liver-fluke. 17 
observers leads us to inquire whether the development of the 
liver-fluke deviates from the type usual among the digenetic 
1 rematoda with whi( h it is always classed, or whether there is 
any cause for failure in the line of experimentation. 
The genera of Trematoda in which the existence of a sporosac 
or nurse-form has been proved, are Monostoma, Amphistoma, 
Distoma, and Gasterostoma. Among Trematodes the ectoparasitic 
Poh/stomeae present no alternation of generations, the egg is 
large, and the embryo produced from it already resembles the 
adult and develops into it by a simple metamorphosis. Holo- 
stoma, which is usually classed with the digenetic Trematodes, 
is in the manner of its development intermediate between the 
digenetic DistoinidcB and the Fohjstomece. The egg is of large 
size, and the embryo passes through an encysted larval state in 
an intermediate host, and the larva is known under the names 
of Diplostoma and Tetracotiile — i.e. there is no alternation of 
generations, although there is a stage passed in an intermediate 
host. But here, as among the Polystomece, the embryo, when 
hatched, is more highly organised than those of the true digenetic 
Trematodes. The embryo of Holostoma cornucopice, according 
to von Linstow,* has a ciliated cuticle and eye-spots, as exist 
in so many Distoma embryos, but otherwise is already very like 
a Tetracotyle. The genus Fasciola differs but little from the 
typical genus Distoma, and although the egg in absolute size is 
almost the largest, if not the largest, among the Distomidae, 
it is by no means the largest relatively, and there is nothing 
in the organisation of the embryo to encourage the idea that 
its development differs greatly from the typical course, or that 
it can be direct. Eggs of the liver-fluke have been given to 
sheep by Professor Simonds,f Gerlach, and other observers, 
and they have never incurred the rot. But these eggs appear 
to have been fresh, and in one case are stated to have beeiL 
so, whereas a prolonged stay in water, at a lower tempera- 
ture than that of the sheep's body, is necessary for the develop- 
ment of an embryo. I accordingly hatched out numbers of 
embryos, and administered them, together with eggs still con- 
taining embryos, to rabbits, but subsequent examination of the 
rabbits found them completely free from flukes. 
It has already been pointed out that the sporosacs of Trema- 
todes are only known to occur in molluscs and fish. It is con^ 
ceivable that they occur, though they have hitherto escaped 
detection, in other animals which inhabit water or moist earth, 
and it may be well to extend our inquiries in this direction. 
* ' Archiv fur Naturgeschichte,' 1875, p. 187." 
t ' The Kot in Sheep,' p. 17. 
VOL. XVII. — S. S. 0 
