INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 357 



tubes, which can be protruded only about an inch in specimens of the 

 ordinary size, and as they depend upon one of these to bring them both 

 food and oxygen, and on the other (dorsal) one to carry off the waste 

 water and excretions, it is essential for their happiness that the orifices 

 of these tubes should be at or near the opening of the burrow most of 

 the time. In this respect the common ''.long clam," Mya arenaria, (fig. 

 179,) and many others that have very long and extensile tubes have a 

 great advantage. But the " razor-shell " makes up for this disadvan- 

 tage by its much greater activity. Its foot, or locomotive organ, (see 

 fig. 182,) is long and very muscular and projects directly forward from 

 the anterior end of the shell ; at the end it is obliquely beveled and 

 pointed, aud it is capable of being expanded at the end into a large bulb, 

 or even into a broad disk, when it wishes to hold itself firmly and se- 

 curely in its burrow. In excavating its burrows it contracts the end of 

 the foot to a point and then thrusts it beneath the surface of the sand ; 

 then, by forcing water into the terminal portion, it expands it into a 

 swollen, bulbous form, and thus crowds the sand aside and enlarges the 

 burrow ; then, by using the bulb as a hold-fast, the shell can be drawn 

 forward by the contraction of the foot; the latter is then contracted into 

 a pointed form and the same operations are repeated. The burrow thus 

 started soon becomes deep enough so that the shell will maintain an up- 

 right position, when the work becomes much easier and the burrow 

 rapidly increases in depth. The " razor-shell," like all other bivalves, 

 depends upon the minute infusoria and other organic particles, animal 

 and vegetable, brought in by the current of water that supplies the gills 

 with oxygen. It is preyed upon by several fishes that seem to be able 

 *o root it out of the sand, or perhaps seize it when at the surface. In 

 this region its principal enemies are the tautog and skates. The latter 

 appear to eat only the foot, for in their stomachs there are sometimes 

 many specimens of this organ, but no shells or other parts. 



The common " long clam," Mya arenaria, (p. 309, Plate XXVI, fig. 

 179,) is also found on sandy shores from low-water nearly up to high- 

 water mark, but it prefers localities where there is more or less gravel 

 or mud with the sand, so as to render it compact, and it has a decided 

 preference for sheltered localities, and especially abounds on the shores 

 of estuaries where there is a mixture of sand, mud, and gravel. It will, 

 therefore, be more particularly mentioned among the estuary species. 

 Yet it is often found even on the outer ocean-beaches, in favorable lo- 

 calities, but not in the loose sands. It lives in permanent burrows, and 

 on account of its extremely long siphon-tubes, which can be stretched 

 out to the length of a foot or more, it is always buried at a considerable 

 depth beneath the sand. The specimens of this shell that live on the 

 outer sandy beaches are much thinner, whiter, and more regular in form 

 than those found in the estuaries ; they are often quite delicate in text- 

 ure, and covered, even when full grown, with a thin, yellowish epidermis, 

 and look so unlike the homely, rough, and mud-colored specimens usually 



