101 
arm with an anterior sucker having a projecting rim and leading 
ito a sac-like intestine which is coloured a bright red and yellow, 
he young redia is not so distinctly coloured and has a collar and 
:ooves across the anterior end of its body. The full-grown redia? 
mtain the cercariffl in different stages. At first no structure can 
3 made out, the cercaria is merely a granular mass ; next it has a 
jl and its head becomes somewhat heart-shaped ; then the 
lokers are formed and the pharynx, intestine and excretory organs 
2gin to appear. These agree in size and position with the similar 
'guns in the encysted form and the size is about the same, 0*7 mm. 
tog. The armature is not developed until the cercaria is nearly 
ill-grown ; the spines round the head first appear and then the 
nail spines which surround the body, except at the posterior 
id. The head spines are 2!) in number, agreeing with the encysted 
inn, and their arrangement is also the same-. The liberated cer- 
uria is very active and contractile, lashing its tail continually, which 
■gan, however, comes off at the slightest touch. The excretory 
•stem is plainly seen, and in living specimens a posterior excretory 
ulb which soon disappears after death. I think this is the Cercaria 
roeima of M. Ch. Lespes (Ann. des Sci. Nat. Ser. 4, Tom 7, lfc>o7) 
Inch in many ways agrees with my specimens. His, probably, 
ere not so far advanced, and the spines were not developed. He 
jund it in the liver of Littorina littorea, only one out of 200 which 
3 examined being infected. He describes it, however, as having 
ae suckers almost equal in size and he found small spines in the 
gal sucker, which I have not been able to make out in my specimens, 
ad in these the posterior sucker is decidedly the larger of the two. 
he bilobed intestine in his figure is much more plainly seen than 
i any of those I found. 
The tailed cercaria presumably leaves the host, swims in the 
ater and is carried by the current into the mussel or cockle, bores 
s way into the foot, settles down and encysts. The mussel or 
)ckle is then eaten by a bird, Oyster Catcher or Herring Gull (both 
I which are known to eat mussels) and in the intestine develops 
»to the adult worm. I hope soon to be able to make some ex- 
ttinients as to the infection of the mussel with the worms from 
te periwinkle. 
Assuming the forms in the periwinkle, mussel, and cockle, and 
ie above mentioned birds to be the same species, we have the 
Wowing life history : — 
