One of the marine mussels (Lithophaga) is a tropi- 

 cal animal with a very different means for boring into 

 rock. It secretes an acid that attacks limy materials, 

 and uses this chemical means to excavate a place for 

 itself. Liiliophaga (the "rock-eater") protects its 

 own shell by a thick brown horny material whose 

 color, along with the shape of the shell, gives the ani- 

 mal the common name "date mussel." 



At Pozzuoli, near Naples, Italy, visitors are shown 

 the work of date mussels as conspicuous pits in the 

 upright limestone pillars of the temple of Serapis. 

 This edifice was built on dry land, and it now stands 

 on dry land, some little distance above the waters of 

 the Mediterranean. Yet in historic times, between 

 the days of Roman civilization when the temple was 

 constructed and the recent past, the land support- 

 ing the building must have subsided enough to let the 

 sea cover the pillars, permitting Lithophaga to pit the 

 surface. More recently the land has risen again, put- 

 ting an end to the erosive action of the moUusks. 



The burrowing habits of pelecypods reach an ex- 

 treme in the wormlike borers that cause so much de- 

 struction to wharf timbers and hulls of wooden ships. 

 The shipworm Teredo navalis may be as much as 24 

 inches long when fully grown, yet its shell consists 

 only of two little Vi -inch plates used as boring tools. 



The minute swimming larva of the shipworm set- 

 tles on a submerged timber and there transforms into 

 the shell-bearing clam. It bores into the wood and 

 continues to do so for the rest of its life. It becomes, 

 in fact, a captive in its own burrow, for as the animal 

 grows it scarcely enlarges the original opening. In- 

 stead, the shipworm excavates a bigger and bigger 

 tube for itself deeper and deeper into the timber, and 

 maintains connection to the sea only by way of two 

 thread-sized siphon tubes. One of these brings in 

 food particles and oxygen. The other discharges 

 wastes (including powdered wood) and reproductive 

 products. Teredo usually follows the grain of the 

 wood, turning aside only to avoid a neighboring ship- 

 worm or a knot. Eventually the timber breaks apart, 

 letting a dock collapse or a ship founder. 



THE AGILE-FOOTED CLAMS 



It would be difficult to find a greater contrast 

 based upon a single body plan than between a ship- 

 worm grinding away, self-imprisoned in a timber 

 from which it gets no nourishment, and a scallop or a 

 cockleshell skipping through the water. The cockle 

 (Cardiuin) is famous for the brittleness of its shell, 

 which is obliquely spherical with rounded ribs. With 

 its long and agile foot the cockle kicks itself along the 

 bottom. Often it draws attention to itself, and a fish 

 swallows it whole or a beachcomber picks it up. 

 Cardium magnum of the Atlantic coast of America 

 reaches 5 inches in diameter on shores from Vir- 

 ginia to Brazil. C. corbis of Pacific coasts is only 



slightly smaller. In both, the extended foot is about 

 as long as the greatest length of the shell. 



A very different means of progression is found in 

 the little fingernail clams of fresh water. The stouter 

 ones (Sphaeriiim) and thinner kinds {Miiscidium) 

 both live in pools of small streams and produce light- 

 weight shells seldom more than Vi inch across. The 

 foot is very slender and adhesive. With it the animal 

 can glide up or down a plant stem as smoothly as 

 though it were a snail or a flatworm. 



The larger clams of fresh water plow a furrow in 

 the bottom, leaving much of the shell exposed. The 

 bladelike foot is extended well ahead of the body, 

 diagonally downward into the mud. Into the tip of 

 the foot the clam pumps blood, producing a knoblike 

 swelling as an anchor. Then it shortens the foot by 

 muscular contraction. If the anchor holds, the clam 

 draws itself forward and downward a little. As the 

 foot is made slender again and extended for another 

 move, the clam's shell may be raised slightly. In 

 consequence, the furrow of the fresh-water denizen 

 usually has an almost regular pattern of greater 



Coquina shells are also called wedge shells or butter- 

 fly shells. This species of clam, Donax rariahilis. is 

 only 1 inch or so long, occurs abundantly on sandy 

 beaches from North Carolina to Florida, and is used 

 to make soup. ( North Carolina. Ralph Buchsbaum ) 



