M. V. Lebour 
417 
loses its larval organs and degenerates into a sac, the sporocyst. From 
the cells of the walls of the sporocyst are budded off germ balls which 
usually grow into a different kind of larva, the I’edia, which bursts from 
the sporoc 3 'st and leads a free existence in the host. The sporocyst 
may however by budding proliferate without forming rediae and may 
form sporocysts within it instead of rediae. The sporocyst is merely a 
sac and has no alimentary canal, imbibing its food through the body 
wall; the redia on the contrary has a well-developed sac-like intestine 
and a mouth at the anterior end with a strongly muscular pharynx. It 
may also have a thickening round the anterior end of the body, the 
“ collar,” a birth-pore usually near the pharynx as exit for the cercariae, 
and at the posterior end two ambulatory projections which help it in 
progression. These latter organs may be entirely absent and frequently 
the ambulatory processes are only present in the young forms, probably 
because the redia migrates in its young state to some other organ of its 
host. Both sporocyst and redia may be coloured yellowish or a brilliant 
orange ; the intestine of the redia is often brown. 
The redia, or, in those cases where the redia stage is omitted, the 
sporocyst now forms within it another kind of larva, the cercaria, which 
has the real Trematode form and gradually grows within its “ nurse ” 
from a shapeless mass of cells to a worm which shows more or less of 
the adult characteristics. Typically the cercaria has a tail and when it 
is full-grown it emerges from the redia or sporocyst. If the birth-pore 
is present in the redia the cercaria comes out in this way without 
rupturing the redia body wall, in the other case it bursts through the 
wall which closes up after it. All stages maj^ be in the redia or 
sporocyst at the same time, from shapeless germ balls to the full-grown 
cercaria. The tailed cercaria now leaves its host altogether and swims 
about in the water for a little while. It then either comes to rest on 
some inanimate object {e.g. a blade of grass as in the Liver Fluke) and 
losing its tail forms round itself a cyst and rests, or it makes its way 
into another host, the second or intermediate host, and here it rests, 
usually forming round itself a cj^st (in exceptional cases such as the 
“Pearl Trematode” a cyst is formed round the cercaria not by the worm 
itself but by the epithelium of its molluscan host). Up to a certain 
point the cercaria may feed and grow in its second host but afterwards 
it alwa 3 ?s has a real resting stage which is comparable to the pupal 
stage of a butterfly. The suckers, alimentary canal and excretory 
system are usually formed in the cercaria before it leaves its first host 
and parts of the reproductive system are sometimes developed. The 
