42 
muscle, in nearly every case caused by Polydora. Frequently the 
posterior adductor muscle was almost destroyed in this way (see 
Fig. III.). Although the remainder of the animal generally 
appeared healthy, this must eventually be a cause of death to the 
mussels, relying as they do so much on these tissues in closing 
their shells when danger threatens and for use of the byssus and 
foot. In some cases, where these pearly masses project and press 
upon the mantle, the reproductive lobes, present elsewhere, are 
quite absent in patches, leaving perfectly clear spaces. Sections of 
the mantle show the entire absence of reproductive lobes in these 
clear patches (see Fig. IV., section of mantle of mussel showing 
reproductive lobes on the right and their complete absence on the 
left where the pearly excrescences have pressed). Here again 
one sees the deteriorating effect of Polydora on the mussel. The 
worm enters from the outside — the small holes the size of a pin- 
prick can often be seen on the mussel shell, — it then makes a U 
shaped burrow, which sometimes, but not always, reaches the 
nacreous layer of the shell, and in two cases it had come right 
through to the inside, the tubes of the worm hanging out into 
the space between the shell and the mantle. In one specimen four 
worms had pierced through, but this is unusual. Polydora is 
nearly always the cause of the pearly excrescences within the 
mussel shell, but in one case a small mussel was found to be the 
cause of a large excrescence. The mussels lower down in the 
Fig. IV. 
