tide. The Coquina Clam, Donax, is faced with much the same problem of making the most of high tides but, in contrast to the sedentary life of its oyster cousin, it leads a very active existence on the sandy beaches along the open ocean. It is an attractive sight when a scouring wave suddenly studs the white beach with dozens of brightly hued clams. The tumbling motion and sudden exposure to light act as a stimulus to the clam which instantly thrusts out its small muscular foot and rapidly pulls itself down into the sand again. During the three or four hours in which the waves are sweeping the middle and upper sections of the beach, the tiny clams may be uncovered and obliged to burrow down again several hundred times.
While many clams prefer clean sand as a habitat, others are habitual mud-dwellers. The handsome Angel Wing, Barnea costata, is usually found in mud so soft and deep that Florida collectors find it extremely difficult to reach them. The Angel Wing is usually located one or two feet below the mud surface and maintains its connection with the bay’s waters with its long siphon. Because of its popularity as a souvenir and collector’s item, methods have been devised to collect them at high tide from a boat or barge. Powerful jets of water are forced through hoses, the mud is swept away from the clams, and then hand-nets are employed to gather them. In more shallow regions where a mixture of sand in the bay bottom permits walking, the exposed Angel Wings are gathered by hand at the next low tide.
The majority of marine clams live in a substrate of sandy mud, but a few have become specialized to the extent of making burrows in exceedingly compact clay, as in the case of the Arctic Saxicave, Hiatella arctica, and the False Angel Wing, Petricola. A few groups such as the Date Mussels, Lithophaga, and the Piddocks, Pholas, burrow into corals, other shells or soft rocks such as sandstone and limestone. The shipworms, Teredo and Bankia, are expert at drilling out their long, tube-like homes in wooden planks of ships, wharf pilings, and manila hemp. So too is the Wood Piddock or Martesia.
A large proportion of bivalves are found in shallow water, but many others are typically deep-water dwellers. The bathymetric range for some species may be narrowly defined in the case of certain scallops, Dipper Clams (Cuspidaria) and astartes. On the other hand, some species found in a few feet of water may also occur in depths of over two miles. One species of Abra Clam, Abra profundorum E. A. Smith, has been dredged in the mid-North Pacific at a depth of 2,900 fathoms—over three miles!
HOW THEY GROW
The shelly valves of clams are the product of the fleshy mantle. This thin, leaf-like organ covers the animal as the flyleaves cover the body of a
