M. V. Lebour 
355 
the foot by boring inwards from the outer wall. The epidermis of the 
foot is very tough and it would probably be difficult for the worm to 
enter this way. 
The cyst is colourless, quite transparent and possesses a thin wall. 
The cercaria enclosed within the cyst has now lost its tail and head 
glands but in other respects exactly corresponds with the cercaria from 
the periwinkle. The spines, suckers and excretory system can be seen 
through the cyst. The excretory system is very conspicuous. 
Nicoll (1906 a, 1906 b) has shown that the encysted cercaria is in 
all probability the larval stage of a new species of Echinostomum which 
he names E. secundum. This worm occurs in the Oyster Catcher 
(Hcematopus ostralegus ), the Herring Gull ( Larus argentatus) and the 
Black-headed Gull ( Larus ridibundus). It has been found by Nicoll 
in the intestines of these birds in all stages from small young speci¬ 
mens, agreeing exactly with the encysted cercaria from the mussel, to 
adults of various lengths, the longest measuring 7*3 mm. Although 
E. secundum resembles E. leptosomum Creplin in many ways, it is 
certainly a distinct species. I have found the latter worm in the 
intestine of the Dunlin ( Tringa alpina ) and the Turnstone ( Strepsilas 
interpres ) and, although the specimens were not in a very good state of 
preservation, it was easy to see that they were certainly distinct from 
the worm described by Nicoll. Moreover E. leptosomum agrees with 
the encysted larva from Scrobicularia tenuis and earlier stages from 
Pcdudestrina stagnalis while E. secundum agrees with the cercaria from 
the mussel and periwinkle above described. As I have shown (1907) 
the redise of the two species differ in form. The cercarire agree in 
almost every point except in size and in the fact that the head spines, 
although equally numerous in both species (29), in E. leptosomum are 
all almost alike in size. The same difference is seen in the adults and it 
is constant. E. secundum is also much broader compared with its 
length than E. leptosomum, the oral and ventral suckers are larger and 
the ova are of a much greater size. 
We may, I think, look upon the larval forms from the mussel and 
cockle as the young of E. secundum although this is unfortunately 
difficult to prove. The above-mentioned birds eat the mollusks 
containing the cysts. The cysts dissolve in the stomach and pass into 
the intestine where they grow into the adult forms. The Oyster 
Catcher, which is common at Budle, feeds constantly on the mussels, 
the stomachs being nearly always full of broken pieces of the shells. 
The Herring Gull and Black-headed Gull are also common shore feeders 
at Budle. 
