INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 309 



uuiisually large victim. This cornmou 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, Finnotheres mactilatus. 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 " liorse-muscle," 

 which is the 2Iodiola modiolus^ (Plate XXXI, fig. 237,) lives at extreme 

 low-water mark in the crevices between the rocks, and usually nearly 

 buried in the gravel and firmly anchored in its place. Sometimes it oc- 

 curs in the larger pools, well down toward low-water mark. It is, like 

 the last, a northern species, and extends to the Arctic Ocean and North- 

 ern Europe. It is 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," My a arena, 

 ria^ (Plate XXTI, 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 Saxicava arct'tca, (Plate XXTII, fig. 192.) It is much more 

 abundant farther 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 shells. The only lo- 

 calities on our coast where I 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 Xew England coast lime- 

 stones rarely occur, and they have to be content with such cracks and 

 crannies as they can fiad ready made ; consequently their shells, in 

 growing to fit their i>laces, 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 generally has a red- 

 dish color. The " bloody clams," Scapharca transversa^ (Plate XXX, 

 fig. 228,) and Argina pexata^ (Plate XXX, fig. 227,) are occasionally 



