17 
The common whelk wears down the posterior or siphon end of the mussel 
shell by a rasping action of its teeth. This method scratches the shell, removing 
the epidermis at the edge. 
The round whelk, like the Purpura, bores a hole in the shell. Only three 
shells whose inhabitants have fallen victims to round whelks have been examined 
by the writer. In these the hole is a large oval one, the largest examined being 
4.5 mm. along the greater diameter and 3.2 mm. along the lesser diameter. In 
-each case the hole was made about midway between the anterior and posterior 
ends of the shell near the edge from which the byssus protrudes. 
The echinoderms observed destroying the mussel at St. Andrews were the 
starfish (chiefly Asterias vulgaris and Asterias forbesii) and the sea-urchin 
(Strongylocentrotus droebachiensis). The starfish, when destroying a mussel, 
opens its shell by means of a steady pull exerted on each valve by the tube feet. 
The shell left after the animal has been destroyed is clean and unmarked. 
The sea-urchin has not commonly been credited with attacking mussels. 
Field (1911), in reviewing the various forms which prey on the mussel, makes no 
mention of the sea-urchin. Ganong (1899) mentions that in Europe it is said 
to attack sea-mussels, but, gives no authority for this statement. Wilcocks 
(1884) quotes “Mr. Harding”’ in a list of enemies, including the ‘‘echinus or 
sea-egg,’’ but gives no exact alithority for this reference so it is of little value. 
Scott (1901), in his investigation of the food of the sea-urchin at St. Andrews, 
found that the principal food consisted of sea-weed. He found no evidence of 
the urchins eating any mollusca and summarizes his conclusions after reviewing 
the literature of the subject thus: “Although practically all who have investi- 
gated the food have concluded that the urchins are herbivorous, there is, seem- 
ingly, among zoologists a general belief that they are carnivorous.” 
The sea-urchin is abundant at St. Andrews, and about low-water large 
numbers of them may be seen on the mussel beds. If disturbed they appear to 
have been simply resting on the mussels rather than attacking them. So 
habitually do they haunt the mussel beds, however, that the writer became very 
suspicious that they were feeding on the mussels, in spite of repeated failure to 
obtain proof of this. 
The first evidence that sea-urchins eat mussels was obtained from an urchin 
found clinging to a bag containing small mussels suspended from the float of the 
wharf. This sea-urchin was brought into the laboratory and placed in one of 
the tanks in running sea-water with a number of small mussels. The following 
morning the contents of the digestive tract were examined. One small-sized 
mussel shell was found in it. Subsequently a number of sea-urchins were collected 
from the mussel beds and the contents of their digestive tracts examined immedi- 
ately. This examination failed to reveal any traces of mussels (either of the 
soft parts or of the shell). 
In order to obtain further evidence two sea-urchins were placed in a small 
aquarium in running sea-water with a number of small mussels (8.5 mm.—10.5 
mm. in length). The following day no evidence could be seen in the aquarium 
of any of the mussels having been destroyed. Three more urchins were added. 
The next day the remnants of at least eight mussel shells were found in the 
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