41 
The presence of Poly dor a ciliata at Buclle is a real evil, but one 
which I fear it is impossible to cure. It is a worm well known for 
burrowing into shells and limestone, and its presence is probably 
due to the proximity of the oysters. Polydora burrows much into 
these, and is much larger in them than when inhabiting the mussel 
shells. One measured over 2^ inches in length, and those in the 
mussel are seldom longer than 1^ inches. The interesting point 
about these burrowing worms is that they cause the mussel to grow 
pearly excrescences, which cover the internal surface of the shell, 
sometimes to an enormous extent, especially in the large mussels 
from the bed of the stream, and thus often materially interfere 
with their muscular development. In some cases the byssal 
muscle and the posterior retractor of the foot were hanging by a 
few strands, and great pearly lumps were projecting into the 
Fig. III. 
