M. V. Lebour 
419 
very nearly related, are, absolutely separated in this way, because in some 
of them owing to an abbreviation of the life-history the cercaria has lost 
its tail. In the Spelotrerna group as described below we have a free- 
swimming tailed cercaria in Spelotreuia excellens, cercariae such as 
Cercaria oocysta in which the young forms possess tails but do not use 
them (as encystment takes place in the sporocyst, where they are cast 
off), and finally in Cercaria littorinae-rudis we have most certainly 
one of the Spelotrerna group which appears to lack a tail altogether. 
We are not justified in such a case, 1 think, in placing these cercariae 
in different divisions because of the presence or absence of a tail. 
Again Acanthopsolus lageniformis loses its tail before emerging from 
the redia. As a full-grown cercaria it is tail-less, but it is much 
more closely related to the Echinostomum group than to the tail¬ 
less cercaria group. So far as we know at present the Gymnophallus 
group is the only one in which the cercaria throughout has no tail, 
and this is a very well-marked group; the fork-tailed cercariae {e.g. 
C. dichotoma) however seem to be closely related. It seems to me 
therefore that the tail is not of so great importance primarily in 
classifying cercariae as is the question whether it develops in sporo- 
cysts or rediae and I have therefore made two primary groups: 
I. Developing in sporocysts. Gasterostomum group. Gymnophallus 
group. Cercaria dichotoma. Spelotrerna group. Group of stumpy-tailed 
cercariae. Lep)odora. 
II. Developing in rediae. Cercaria neptimeae. Acanthopsolus. 
Echinostomum group. Monostomum group. 
From this classification I have omitted all fresh water forms that 
are not also marine and have restricted myself to those already known 
in Britain. 
We divide the digenetic Trematodes into Gasterostomata and 
Prostomata according to whether the oral sucker is median or, as is 
usually the case, at the anterior end. This division is as clearly marked 
in the larval forms as in the adults. As well as having the sac-like 
intestine opening in the centre of the body the Gasterostomatous 
cercaria, which is known as Bucephalus, is provided with a peculiar and 
very characteristic tail having two long lateral filaments by aid of which 
it swims. 
The Prostomata may then be sub-divided into groups some of which 
stand out much more conspicuously than others, but this is probably 
because they are more completely known; such groups are the 
Spelotrerna group and the Echinostomum group, both of which are 
