M. V. Lebour 
i 
Plymouth which have not got this parasite, but these were un¬ 
successful. 
The genital gland of Littorina rudis, when this cercaria is present, is 
white and very much swollen ; in L. littorea it is orange-coloured 
although the colour only affects parts of the sporocyst and the tissues 
surrounding them, not the cercariae which are colourless. 
The sporocysts (fig. 6) are long and thin-walled,0‘40mm.-0'80inm. long 
and 0‘20mm.-0’35mm. broad. All were full of cercariae in various stages 
of development. 
The cercaria (figs. 5, 6) is extremely active with a long tail which is 
continually in motion and the whole worm is so constantly changing its 
shape that its structure is very difficult to see when alive. The body 
measures 0'38 mm. in length without the tail, but can be much shorter 
or longer when contracted or expanded. The tail is nearly the length 
of the body. The breadth of the body is about a third of the length. 
The whole surface is covered with minute spines dwindling posteriorly. 
The oral sucker measures 0 03 mm. across and is circular with a circular 
aperture. On its dorsal surface is a .single straight stylet measuring in 
length not quite half the width of the sucker. The ventral sucker 
is peculiarly prominent protruding as a great bulb from the ventral 
surface. It is situated almost exactly in the centre of the body and is 
the same size as the oral sucker or slightly larger. The aperture which 
is round is set at an oblique angle to the body. 
The oral sucker leads to a somewhat obscure alimentary canal with 
no apparent pharynx (Pelseneer however shows a pharynx in his figure so 
my specimens may be younger than his). In some the intestinal caeca 
could he made out, with the fork well in front of the ventral sucker and 
the caeca reaching to just behind the level of the ventral sucker 
and slightly swollen at the ends, reminding one very much of the 
intestine of the encysted Maritrema. 
The excretory vesicle is Y shaped, the forks reaching to the level of 
tlie anterior margin of the ventral sucker. The tail is very transparent 
and extremely contractile with striations both transverse and longitudi¬ 
nal, the transverse being the more conspicuous. 
I think it is possible that this cercaria may prove to be the first 
stage of the Maritrema. from Ligia. but this is as yet not at all certain. 
With regard to the adult stage of this Maritrema it does not agree 
exactly with any known species and is probably a new one. 
The species of Maritrema all inhabit birds and Nicoll (1907) 
describes three British species occurring in the Dnnlin {Peliduaalpina), 
