188 UNSEGMENTED “ WORMS.” 
sporocyst, and migrate into the liver or some other organ. 
Each sporocyst usually forms at a time 5-8 rediaz; each 
of these forms 8-12 more redize; and each of these forms 
14-20 cercariz. In the winter a sporocyst may give rise 
to cercariz directly. A redia is a cylindrical organism 
with a short alimentary canal, excretory canals with “ flame 
cells,” and a pair of blunt locomotor processes posteriorly. 
A cercaria has a bifurcated gut, two suckers, a locomotor 
tail, and the beginnings of gonads (Fig. 97 (6)). 
The cercarize emerge from the rediz, wriggle out of the 
snail, pass into the water, and after swimming for a short 
time, moor themselves to stems of damp grass. There 
they lose their tails and become encysted. If the encysted 
cercaria on the grass stem be eaten bya sheep, the cyst 
is dissolved in the stomach, and the young fluke makes 
its way up the bile duct and its tributaries. In about six 
weeks it grows into the adult sexual fluke. 
It will be noted that the sporocyst is the modified embryo, but that 
it has the power of giving rise asexually to redie. These develop, 
however, from special cells of the sporocyst, which we may compare to 
spores or to precociously developed parthenogenetic ova. Though the 
reproduction is asexual, it is not comparable to budding or division. 
The same power is possessed by the redize, and there are thus several 
(at least two) asexual generations between the embryo and the adult. 
The disease of liver-rot in sheep is common and disastrous. It has 
been known to destroy a million sheep in one year in Britain alone; and 
in the winter 1879-80 the mortality attributed to fluke disease was 
estimated at three millions. It is especially common after wet seasons, 
and in damp districts. 
Classification.—Order 1. Heterocotylea, with a posterior ad- 
hesive organ, often with a pair of accessory suckers beside the mouth. 
Most are ectoparasitic. The development is direct and associated with 
one host (monogenetic). 
e.g. Polystomum integerrimum, This form with many suckers 
is often found in the bladder of the frog. It attaches 
itself in its youth to the gills of tadpoles, passes thence 
through the food canal to the bladder, where it develops 
slowly for years. 
Gyrodactylus, found on the gills and fins of fresh - water 
fishes. It is viviparous, but the embryo, before it is 
extruded, itself contains an embryo, and this in turn 
another, so that three generations of embryos are re- 
presented simultaneously. 
Diplozoon paradoxum consists of two individuals united. 
The single embryo (Dzforfa) is at first free-swimming, 
