TREMATODA. ~ 183 
syncytium (z.e. without distinct demarcation into cells) which 
secretes a-thick cuticle, contains rhabdites, and rarely bears 
cilia. The class seems to be intermediate between Rhab- 
doceelid Turbellaria and Trematodes, 
Class TREMATODA. Flukes, etc. 
The Trematodes are leaf-like, or sometimes cylindrical 
external or internal parasites. With their parasitic life may 
be associated the absence of cilia on the surface of the adults, 
the thick “cuticle,” the presence of attaching suckers (occasion- 
ally with hooks), and the rarity of sense organs. After 
embryontc life the ectoderm degenerates, ceases to be distinctly 
cellular, and sinks inwards. Jt is likely that they have 
arisen from free Turbellarian-like ancestors, and they resemble 
the Turbellarians in being unsegmented, in having anterior 
ganglia, from which nerves pass backward and forward, in 
the rudimentary nature of the body cavity, in the ramifying 
system of fine excretory canals, in the hermaphrodite and 
usually complex reproductive system. The excretory and 
nervous systems are, however, more complex than those of 
Turbellaria. The alimentary canal is usually forked, 
often much branched, and always ends blindly. In many 
cases the animals are self-impregnating, but cross- 
fertilisation also cecurs. The development of the external 
parasites is usually direct, of the internal parasites usually 
indirect, involving alternation of generations. They occur 
on ovr in al] sorts of Vertebrates, but those which have 
an indirect development, and require two hosts to complete 
their life-cycle, often pass part of their life in some 
Invertebrate. 
Type, Zhe Liver Fluke (Distomum hepaticum) 
The adult fluke lives as a parasite in the liver and bile 
ducts of the sheep, causing “‘liver-rot.” Unlike most 
flukes, it has many occasional hosts,—it sometimes occurs 
in cattle, horses, deer, camel, antelopes, goat, pig, beaver, 
squirrel, kangaroo, and rarely in man. The animal is flat, 
oval, and leaf-like, almost an inch in length by half an 
inch across the broadest part, reddish brown to greyish 
yellow in colour. As the word Distomum suggests, there 
