6 
Larval Trematodes 
geuus from Spelotrema, has turned out to be erroneous as we see this 
present form encysting in a Crustacean. 
So far the first larval stage of this species has not been found 
although much search has been made. The first host must almost 
certainly be a mollusc and the only mollusc living with Ligia below the 
laboratory at Millport is Littorina rudis. I have examined a great 
number of these but have not found a cercaria which certainly 
corresponds with this Maritrema. The Littorinae live under the same 
stones with Ligia as well as in small shallow pools and on the rocks 
close by, and also extend further down into the Pelvetia zone and even 
into the zone of Fucus vesiculosus. The Littorina rudis from all these 
parts contained several larval Trematodes the most frequent being the 
sporocysts and cercariae of what I believe to be Spelotrema excellens 
Nicoll, which are perhaps identical with the Cercaria hrevicauda 
of Pelseneer from the same mollusc (Pelseneer 1906). This cercaria 
encysts in the Green Crab (Cam wms moe?ias), which does not live so high 
up as Ligia but does live with Littorina rudis lower down. I tried to 
find differences in these sporocysts and cercariae from the Littorinae 
living with Ligia but failed. 
Three other species of cercariae were found in Littorina rudis living 
with Ijigia: — Cei'caria linearis Lespes. This occurre'd in one specimen 
together with the sporocysts of Spelotrema excellens. The cercariae are 
contained in sporocysts in the kidney whilst those of S. excellens are in 
the digestive gland. This is a new host for this species as hitherto it 
has only been known from the common Periwinkle Littorina littorea 
(Lespes 1857 and Lebour 1912). 
Of the other two, one is a Monostomum cercaria in rediae, probably 
new, and the other is apparently identical with Cercaria emascidans 
Pelseneer (1906). 
Cercaria emascidans }*eheneer. This cercaria occurred in sporocysts 
in the genital gland of Littorina rudis living with Ligia and also in that 
of L. littorea from Balloch Ba}^ Millport. It occurred more frequently 
in L. rudis which is the orginal host for it, L. littorea being new. The 
sporocysts are very like those of Spelotrema excellens and in many ways 
the cercaria resembles a Spelotrema cercaria. This made me think it 
might be the first stage of the Maritrema. The shape and size of the 
excretory vesicle however prevent my being in any way certain of this, 
although it is conceivable that the worm may grow very much larger 
without a proportionate growth in the excretory system. Some infection 
experiments were tried with this larval form and some Ligiae from 
