416 
A REVIEW OE THE BRITISH MARINE 
CERCARIAE. 
By MABIE V. LEBOUR, M.Sc. 
Assistant Lecturer and Demonstrator in Zoology^ Leeds University. 
(With Plates XXV—XXIX.) 
General Life-History. 
The study of the life-histories of digenetic Trematodes is as yet in 
its infancy. Many new adult forms are described every year, but very 
little is known of the life-history of any of these. It is even impossible 
in the majority of cases to say what sort of larval form is characteristic 
of any group. Having worked for some years at the study of Trematode 
larvae I thought it would be of .some use to bring together all the work 
so far done on British marine cercariae in order to form a nucleus round 
which future work may be centred. 
As is to be expected more is known of the larval stages of bird 
Trematodes than of those inhabiting tish, as a great many sea birds are 
shoj-e feeders and it is much easier to find cercariae in the shore animats 
than in those from the sea where the fish feed. It is only with fish and 
bird Trematodes that we deal in the pi’esent memoir. 
From the fertilised digenetic Trematode egg issues a ciliated larva, 
the miracidium, possessing certain larval organs, which is free-swimming 
and after emerging from the egg in the water makes its way into some 
invertebrate host, the first host. This is in almost every case known a 
Mollusk, either Gastropod or Pelecypod. (The only exception known 
to me is that suggested by Pratt (1898, p. 351) who thinks it probable 
that a Hemiurid larva develojDS from a miracidium in a Crustacean 
host, namely a Copepodk) Having bored its way into this host the 
larva settles down in some organ, usually the digestive gland or gonad, 
1 The larva of the human Trematode Bilharzia is apparently capable of direct 
infection. 
