M. V. Lebour 
421 
No prepljaryiix, conspicuous pharynx : short and broad intestinal caeca 
seldom reaching beyond ventral sucker. Ventral sucker always behind 
centre of body. Excretory vesicle large, forked, reaching far forward 
anteriorly. 
Intermediate host usually a Pelecypod. Cercaria lives between 
mantle and shell, finally settling down in the tissues of its host which 
forms a cyst round it of epithelial cells, the cercaria itself not forming 
a cyst. 
Final host a bird, usually of the Duck tribe. 
{b) Group of fork-tailed Gercariae. These seem closely allied to 
Gymnophalhis but further life-history not known. Cercaria very like 
Gymnop)hallus but with excretory vesicle continuing down tail and 
opening at end of each fork. Develops in sausage-shaped sporocysts in 
Pelecypoda. 
(c) Spelotvema group. Cercaria developed in sporocysts, spherical 
or oblong, in Gastropods. May have a free-swimming stage and in that 
case {e.g. Spelotrema excellens) is very small, oval and transparent with 
a thin tail about as long as or slightly longer than the body, provided 
with a boring stylet anteriorly and large stylet glands. If free- 
swimming stage is not present the cercaria is like the adult, tongire- 
shaped, covered more or less with spines, with long prepharynx and 
oesophagus, short intestinal lobes, not reaching to end of body. Tail 
may be completely absent or present in young stages of those which 
encyst in the sporocysts. 
If intermediate host is required it is a Crustacean in which the 
cercaria encysts. 
Final host—a bird. 
{d) Group of stumpy-tailed Gercariae. Cercaria developed in 
sausage-shaped sporocysts, often brightly coloured, in Gastropoda. 
Very small and transparent. Suckers almost of the same size. Oral 
sucker provided with a stylet and conspicuous stylet glands. Excretory 
vesicle very thick-walled. Tail very broad and stumpy. 
Further life-history unknown. 
(e) Lep>odora. Only one species of cercaria known, oval, covered 
with spines. Intestinal lobes reaching nearly to posterior end of body. 
Tail present in young form, cast off before encystment which takes 
place within the sausage-shaped sporocysts in a Pelecypod. 
Final host—a fish. 
