43 
Budle bed to the south-east are not attacked by Polydora nearly so 
frequently, only one or two specimens out of many examined 
contained their excavations, and these only singly, whereas those 
above-mentioned were often quite riddled with them. This fact 
seems to point to the oysters as the cause, for the nearer the 
mussels are to the oysters the more are their shells bored by 
Polydora. Apart from being a cause of death in a slight degree to 
the mussels, the presence of these excrescences is probably a 
disadvantage in those destined for selling for food in the market, as 
they have a most unsightly appearance when opened. 
The larval Trematodes inhabiting the mussel are three in 
number, viz.;* in the mantle the cercaria of the “Pearl Trematode,” 
in the foot the encysted cercaria of Echinostomum secundum (Nicoll), 
and in the liver an encysted form not yet determined. Although I 
have classed them among the enemies of the mussels they appear 
to do very little harm to their host, even when present in large 
numbers. 
The “Pearl Trematode,” which Dr. Lyster Jameson proved to 
be the cause of the mussel pearls,! is the cercaria of an adult form 
living in the Eider Duck. Odhner } is of the opinion that this is 
Gymnophallus bursicola (Odh.), and not G. somateruc [i.e., Leuci- 
thodendrium soma te rice), as Jameson suggests. The tail-less cercaria 
occurs free on the mantle of the mussel before it is covered over 
with nacre. The previous host at Budle is Cardium edule , in which 
it occurs abundantly underneath the hinge in masses of sporocysts, 
but although found so frequently in this stage it is not nearly so 
often seen in the mussels. Pearls, however, occur in numbers at 
Budle, so that if each pearl means a worm, the cercaria must 
succeed fairly often in reaching its second host. An interesting 
mussel was found (see Fig. V.), having a ridge on the mantle 
composed entirely of separate pearls, probably a hundred or more. 
The length of the mussel was 3£ inches, and the ridge was 1£ 
inches long. This may have been due to an inrush of Trematodes, 
apparently a most unusual occurrence. 
None of these worms were found in the mussels from the other 
Northumbrian beds, although there are a good many pearls in those 
from the Holy Island scaup. At Berwick, Amble, and Blyth there 
are very few pearls, and none at all were found in the mussels from 
Alnmoutli. 
• See “ Some Trematodes in Mytilus,” op. cit. 
+ Proc. Zool. Soc., London, 1902, p. 140. 
t “ Die Tremntoden des Arktischen Gebietes, 1905, p. 25. 
