Lamellibranchia

It is stated that certain carnivorous Gastropoda use the spines of teeth situated on the shell-aperture for forcing open the shell of Lamellibranchs. Thus a species of Murex is said to grasp an Arca with its foot and to drive one of its spines between the valves of the latter. Sycotypus is said to use the edge of its own shell for this purpose.

The Lamellibranchia are regularly or periodically exposed to other dangers. The special risks which are run by forms living in sand on open coasts have already been described. Freshwater forms in most parts are regularly exposed to the dangers of low or high temperature. The water in which they live may be frozen over, its temperature may be lowered or it may be dried up by excessive heat. These dangers are met in most cases by the habit of hibernation or of aestivation.

The Lamellibranchia are probably as tenacious of life as Gas tropoda (q.v.) though less is known concerning them in this re spect. An Australian pond mussel is recorded as having lived for nearly 500 days out of water.

Commensalism, or the food-sharing association of one non parasitic animal with another, is of rather common occurrence among Lamellibranchs. Lepton squamosum lives thus in the bur rows of Crustacea and marine worms. Jousseaumia inhabits the chamber of a Sipunculid and species of iliontacuta and Scioberetia live on Echinoderms. Other bivalves are more closely associated with certain animals. Modiolaria marmorata lives embedded in the test of Ascidians and Vulsella in the tissues of sponges. Entovalva is parasitic in Synapta.

The age to which these animals live varies considerably. Mytilus and Cardium attain their full size in a year, the common oyster in five years. The latter is said to live as long as ten years under cultivated conditions. The giant clam (Tridacna gigas) lives for eight years (probably a very low estimate). The swan mussel (Anodonta) is recorded as attaining an age of 20-30 years and Weymouth has calculated that the pismo clam (Tivela stultorum) lives as long as 26 years.

Like the Gastropoda among other Mollusca the Lamellibranchia have supplied man with food since a very early stage in his evolu tion. Oysters, cockles, mussels and clams have been found ib

large quantities in Palaeolithic and Neolithic middens in Europe and elsewhere. The oyster was cultivated under the Roman re public, the celebrated beds in the Lucrine lake being established about ioo B.C. During the empire British oysters, which were very much in favour, were brought from Rutupiae (Richborough) where there was apparently a fishery. At the present time oysters are cultivated in the British Isles, France, the United States, Aus tralia and elsewhere. The principal English beds are at Whit stable, Colchester, Burnham-on-Crouch and Conway (see OYSTER). The cockle, scallop and mussel are likewise caught for human consumption in European countries, and a variety of other forms (including those of freshwater) are eaten in most parts of the world. In the Gulf of Naples Psammobia, Solen, and Donax are fished for the market. The importance of Lamellibranchia as food for edible fishes has already been discussed.

The shells of certain genera are regularly cultivated for the mother-of-pearl which is used for inlaying, various kinds of orna knife-handles, buttons, etc. The Orient pearl oyster Melea grina margaritifera is fished for this purpose and for the pearls of various kinds obtained from it. The most important pearl fisheries are in Ceylon, Australia and Japan. In the latter country pearls are produced artificially in the oyster by a grafting opera tion (see PEARL). From Roman times onwards the pearls of the European freshwater mussel (Margaritifera margaritifera) were sought especially in Scotland, though this industry no longer exists. The large river mussels (Unionidae) of the United States are largely cultivated in connec tion with button-making.

Shell-money is made from clams in various parts of the world and the "wampum" of eastern North America consisted largely of beads made from the shell of Venus mercenaria.

In southern India and parts of China the "window-pane oyster" Placuna placenta is still used for glazing the windows of houses. The byssus of the large species of Pinna was at one time used as a substitute for silk in southern Italy.