16 
were not found along considerable stretches of the shore as then pointed out. 
In 1918 the writer found that practically all parts of the shore possessed a number 
of mussels, even unfavourable locations (e.g., gravelly shores) possessing an 
appreciable number. In view of the rate of increase in length of the shell of the 
sea mussel per growing season at St. Andrews, viz., 10.8 m. these facts indicate 
that the mussels of the region were practically wiped out by some unfavourable 
condition and that in 1916 and the following seasons they were re-inhabiting 
the region. 
2. Age of Mussel Beds at Digby, Nova Scotia. 
The shells of mussels collected in the survey of 1918 at Digby, Nova Scotia, 
have been studied in order to determine the rate of growth for that region. 
This study revealed that the average rate of increase in length of the shells 
per growing season is 16.0 mm. It will be seen, therefore, that the larger sized 
mussels of from 6 cm. to 8 cm. inJength are in their fourth, fif-h or sixth growing 
season, the age depending on whether or not the conditions which had obtained 
during their various growing seasons were favourable. 
SECTION -Iit 
ENEMIES OF THE SEA-MUSSEL AT ST. ANDREWS, N-B: 
During the free-swimming larval stage of the sea-mussel it forms part of 
the plankton and consequently large numbers are undoubtedly destroyed by 
those forms which feed on the plankton. After becoming well-established in 
some suitable location for growth it is still subject to the attacks of several 
animals. At St. Andrews, fish, other molluscs and echinoderms are the chief 
offenders. A few mussels are probably destroyed by the gulls and crows which 
occasionally frequent the beds at low-tide. 
Fish destroy a large number of mussels. Professor A. G. Huntsman (1920) 
informs me that they form “‘a not inconsiderable part of the food of the flounder 
(Pseudopleuronectes) at St. Andrews,” and results obtained by Clemens and 
Clemens (1921) in their study of the eel pout (Zoarces) at St. Andrews show that 
in an examination of 75 specimens, 12 of them (z.e., 16%) were found to contain 
sea-mussels, the largest number found in a single specimen being 122. It seems 
probable that further study will show that other fish prey on the mussels in this 
region. 
The molluscan enemies observed devouring the mussels at St. Andrews are: 
Purpura lapillus, Buccinum undatum (common whelk)- and Polynices heros 
(round whelk or ‘‘cockle’’). Each one of these attacks the mussel in a somewhat 
different way. The Purpura bores a small round hole about the size of a common 
pin head in the shell and thus gains access to the animal within. These holes 
are made in any convenient portion of the shell. Very frequently they are 
found near the umbo. If the Purpura, while attacking a mussel, is disturbed 
before the animal is injured the mussel appears to plug the inner end of the 
hole bored by the Purpura with a pearly excretion, since shells have been found 
with small holes (considered to have been made by a Purpura) which have been 
plugged in this manner. 
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