[309] INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 15 
unusually large victim. This common muscle is not only useful to man 
directly as food, and as a fertilizer, but it serves as an important article 
of food for many fishes, both in its young stages and when full grown. 
The tautog makes many a hearty meal on the full-grown shells, as do 
several other kinds of fishes, while the “scup” and others devour the 
young. The common star-fishes feed largely upon muscles, as well as 
oysters, and, they also have many other enemies.’ A small parasitic 
crab, Pinnotheres maculatus, lives in their shells, between their gills, 
in the same manner as the common Pinnotheres ostreum lives in the oys- 
ter. Another larger muscle, sometimes called the “ horse-muscle,” 
which is the Modiola modiolus, (Plate XXX, fig. 237,) lives at extreme 
low-water mark in the crevices between the rocks, and usually nearly 
buried in the graveland firmly anchored in its place. Sometimes it oc- 
curs in the larger pools, well down toward low-water mark. 1t is, like 
the last, a northern species, and extends to the Arctic Ocean and North- 
ern Europe. Itis much more abundant on the northern coasts than 
here, and, although it is almost entirely confined to rocky shores and 
bottoms, it extends to considerable depths, for we dredged it abun- 
dantly in the Bay of Fundy, at various depths, down to 70 fathoms. 
Like the preceding, it is devoured by the tautog and other fishes. Its 
thick shell, covered with a glossy, chestnut epidermis, and rudely hairy 
toward the large end, are points by which it can easily be recognized, 
and its shape is also peculiar. The common “long clam,” Mya arena- 
ria, (Plate XXVI, fig. 179) is very often met with buried in the sand 
and gravel beneath stones and rocks, but it is far more abundant on 
sandy and muddy shores, and especially in estuaries, and will, there- 
fore, be mentioned with more details in another place. 
Another shell, somewhat resembling the “long clam,” but never 
growing so large, and more cylindrical in form though usually much 
distorted, is occasionally met with under the rocks or in crevices. This 
is the Savicava arctica, (Plate NXVII, fig. 192.) It is much more 
abundant fartber north, and has a very extensive range, being found on 
most coasts, at least in the northern hemisphere. On those coasts 
where limestone exists it has the habit of burrowing into the limestone, 
after the manner of Lithodomus and many other shelis. The only lo- 
calities on our coast where [ have observed this habit are at Anticosti 
Island, in the Gulf of Saint Lawrence, where the soft limestones are 
abundantly perforated in this way. On the New England coast lime- 
stones rarely occur, and they have to be content with such cracks and 
crannies as they can find ready made; consequently their shells, in 
growing to fit their places, become very much distorted. This species can 
also form a byssus, when needed, to hold its shell in position. The 
siphon-tube is long and much resembles that of Mya, (see fig. 179,) 
but is divided at the end for a short distance, and gererally has a red- 
dish color., The “bloody clams,” Scapharca transversa, (Plate XXX, 
fig. 228,) and Argina pexata, (Plate XXX, fig. 227,) are occasionally 
