4 68 
WORMS. 
perish. Some however, in all probability, make their way into water. When this 
has taken place, the egg, after two or three weeks, gives birth to a free-swimming, 
ciliated, conical embryo, provided with a double eye and rudiments of an excretory 
system. By means of its cilia, this embryo swims 
rapidly about in search of a particular species of 
pond-snail. If it fails in its search, it perishes in 
about eight or ten hours; but, if successful, it proceeds 
to bore its way into the soft tissues of the mollusc. 
As soon as it has effected an entrance, it loses its cilia 
and turns into an oval sac, the sporocyst. The latter 
may multiply by fission, but in any case, in its 
interior, another organism, called after Redi, its 
discoverer, Redia, arises. This bores its way out of 
the sporocyst, which closing up again forms another; 
but if too many are developed they may cause the 
death of the snail. The Redia is cylindrical in shape, 
and has a distinct mouth and stomach, and in the 
hinder half of its body there is a pair of bud-like 
processes, serving as rudimentary feet. The larva 
in this stage takes up its abode in the liver of the 
snail, where, in turn, it proceeds to propagate. Its offspring may be a Redia like 
itself but more often it has a different form, and has received the name Cercaria. 
LARVAL FORM OF LIVER-FLUKE (magnified). 
echinatum. 
It escapes from the parent Redia by an aperture situated near the front end of 
its body; and presents a considerable resemblance to a tadpole, consisting of a long 
vibratile tail, and a wide heart-shaped body with a forked intestine, two suckers, 
