M. V. Lebour 
433 
cyst without injuring it as the cysts are extraordinarily difficult to 
break through. 
The cercai'ia when pressed out of the cyst measures about 0'8 mm. 
in length. Its body is covered with spines except at the postex'ior end. 
Oral sucker 0'066 mm. across, slightly larger than the ventral sucker. 
Prepharynx long, pharynx conspicuous, oesophagus long branching into 
two caeca which reach to about the level of the centre of the ventral 
sucker. The genital papilla is plainly to be seen on the left side of the 
ventral sucker and often the vesicula seminalis is apparent. The 
excretory vesicle is large and specially conspicuous in the hind part of 
the encysted cercaria, two lateral ducts are plainly to be seen opening 
into it. Testes one on each side behind ventral sucker. From what has 
been said there can be little doubt that Cercaria uhiqidta is the young 
form of S. excellens the first host thus being Paludestrina stagnalis, 
Littorina ohtusata and L. rudis. The intermediate host Carcinus 
maenas and Cancer j^gtiriis and the final host probably the herring 
gull Larus argentatus. 
The sporocysts and cercariae are not to be found in the winter and 
the crabs in the winter only contain the thick-walled cysts. The small 
thin-walled cysts are only to be found in the spring, summer and early 
autumn. This points to the summer and autumn as the time for the 
migration of the cercariae from the first to the intermediate host. The 
passing from Carcimis to its final host may almost certainly take place 
at any time of year. 
The cercariae C. cellidosa and C. pusilla of Looss (1896, p. 227) 
probably belong to the Spelotrema group as they are very like Cercaria 
ubiquita. In that case the final host would be a bird and not an 
insectivore as the author suggests. 
There are at least two other forms belonging to the Spelotrema 
group which encyst in the crab but which I have not been able to 
investigate in any detail. These are Cercaria carcini mihi and another 
small one which I have named Cercai'ia minor. The encysted cercaria 
with thick cyst mentioned by m 3 "self from Carcinus (1908 a, p. 11) is 
probably an unusually small example of S. excellens. 
