M. V. Lebour 
431 
therefore it is hardly possible to ascribe them to different genera. The 
genera Spelotrema, Maritrema, Levinsenia, Levinsiella are very much 
alike and are only differentiated by their reproductive and excretory 
organs. It is sometimes easy to see the genital opening and papilla in 
the Spelotrema cercaria, and in one species 8. excellens Nicoll I have 
been able to follow out most of the life-history. 
Some forms of this group encyst in their sporocysts in Gastropod 
hosts and these I think may possibly belong to the genus Maritrema. 
There are two distinct kinds of cercariae in the Spelotrema group. 
One in which the cercaria has a free-swimming stage and is provided 
with a stylet and stylet glands. This is at first quite unlike the 
encysted form and gradually assumes the adult characteristics within 
the cyst. Such cercariae occur in S. excellens and S. halani. The 
other kind is like the encysted form even within the sporocysts, 
apparently possessing no tail at any period of its existence. This is the 
case with Cercaria littorinae-rudis. In such forms which encyst in the 
sporocyst the cercaria is at first tailed but gradually assumes the 
ordinary Spelotrema form, loses its tail and encysts. 
Genus Spelotrema Jagerskibld. 
Spelotrema excellens Nicoll (x. 1907, p. 248). 
(PI. XXVI, figs. 3-10.) 
The encysted cercaria which for excellent reasons Nicoll regards as 
S. excellens occurs in the common green crab Carcinus inaenas in almost 
every organ and seems to be universally distributed in Britain. The 
first to describe this cercaria from the crab was M'^Intosh (1865, p. 201) 
who found it abundantly at St Andrews from the crabs in the rock 
pools. It was afterwards described by Nicoll and Small (1909, p. 238) 
from Millport, and I have found it commonly at Plymouth and on the 
Northumberland coast. 
The first host of this Trematode has not as yet been described but I 
believe the very active and extremely small cercaria which is common 
in many places and which I have named Cercaria ubiquita (1907, p. 444) 
to be a stage in the life-history. It never occurred to me to identify 
this little creature with the encysted Spelotrema from the crab until 
after pondering over the question of its life-history at Millport and 
examining many Mollusca and crabs I was able to follow up the various 
stages of Cercaria ubiquita and found that it entered the crab, and there 
underwent changes until it finally turned into a Spelotrema-Wke cercaria. 
