FLAT-WORMS. 
467 
Two-Suckered Group,—S uborder Distomese. 
Polystomum and larva (magnified). 
The second division of trematodes is dis¬ 
tinguished by the smaller number of suckers, 
the absence of hooks, and the circumstance 
that all the members of the group are internal 
parasites, laying an immense number of small 
eggs; while in the course of their development 
the young are inhabitants of more than one 
host. It is evident that parasites living upon 
the skin or gills of fish, where they are constantly in danger of being washed 
■away, have much greater need of sucking-discs and clinging-hooks than those 
living within shelter of some internal organ. On the other hand, it is equally 
clear that the large number of eggs laid by the 
internal forms, which pass through a complicated 
metamorphosis, is a means for providing against the 
remoteness of the chances of the larvee meeting with 
their appropriate hosts. Some of these worms are of 
importance, on account of the destruction they bring 
upon the hosts they infest. One of the best known 
is the liver-fluke (Distomum hepaticum), found in 
the mature stage in the livers of sheep. It is about 
an inch in length, and nearly half an inch broad. 
The hinder portion of its body is flattened and leaf¬ 
like, but the front is thick and conical, and the outer 
skin is furnished with many backwardly-directed 
•spines. The eggs—of which it has been computed half a million may be laid at a 
time—pass into the intestine of the sheep by way of the bile-ducts, and thence make 
their way to the exterior. Many of these eggs fall upon dry ground, where they 
ISr 
liver-fluke and larva (enlarged). 
eight weeks to hatch; and the young worm is an active little animal, swimming 
by means of the cilia with which its body is bordered. It differs from the adult by 
the presence of the fringe of cilia which extends along the sides of the body from 
head to tail, and also by the absence of suckers 
at the posterior extremity of the body. The 
latter, however, is furnished with eight pairs of 
hooks, which are retained in the adult. After 
leading a free life for a short time, the larva 
attaches itself to the external gills of a tadpole, 
and speedily loses its clothing of cilia. When 
the gills shrivel with the conversion of the 
tadpole into a frog, the larva enters the mouth 
of its host, and, passing thence into its intestine, 
succeeds in ultimately making its way into 
the bladder, where it lives some five or six 
years before reaching maturity. 
