HOW THEY SENSE AND SEE
Bivalves are the least “brainy” of the mollusks and, although the central nervous system forms a rather complicated latticework throughout the body, its three pairs of “brains” are merely swellings or ganglia in the larger nerves. The pair of so-called cerebral ganglia control the actions of the lip palps near the mouth, parts of the mantle, and they also receive “nerve notices” from the tiny organs of balance, the otocysts. The second major pair of ganglia are the pedals which supply the foot. This pair is large in the clams that use the foot for digging or burrowing, but it is extremely small or aborted in the oysters in which the foot is not used. The third pair, or visceral ganglia, is usually the largest and supplies the adductor muscles and the visceral mass. The remarkable eyes of the scallops are connected with this pair of visceral ganglia.
Many of the bivalve larvae possess true paired eyes, but in all cases these are lost when the animal transforms into the adult stage. The adults of a number of clams and mussels have developed pigment spots sensitive to changing light, but in the scallops true eyes are well-developed. When the shell of a scallop is open there can be seen just within the margin of each valve a line of small, brilliant, emerald-like dots on the mantle, each of which is a small eye fully equipped with cornea, focusing lens, receptive retina and conducting nerves.
HOW THEY BURROW AND SWIM
There are bivalves that swim, leap, crawl and burrow deeply in mud, sand or clay, and some that bore into wood, rock and even lead casings of submarine cables. Even the rock-bound oyster and the stuck-in-the-mud clam have their days of wandering about as free-swimming larvae before they settle down to a life of permanent attachment or clumsy crawling.
The habit of swimming among adult bivalves is rare. The scallops and the Lima File Clams not infrequently swim. Only under the abnormal condition of finding themselves “unearthed” do the Ensis Razor Clams and the Solemya Veiled Clams practice jet propulsion through the water. The Razor Clam swims backward in quick, short jerks by first extending its long cylindrical foot out from the shell and then suddenly withdrawing it with great force. This action, together with the closing of the shell valves, quickly forces the water within the mantle cavity out through the openings at the anterior or foot end. Thus the razor clam darts through the water with its pumping foot to the rear. In Solemya, the foot is in front of the animal as it swims. In this case the water is admitted around the foot but is expelled from the opposite end through the siphons.
