The Cardiacea

The Cardiacea consists of two families only, the Cardiidae and the Tridacnidae. According to Theile, the former comprises eleven genera, and is of world-wide distribution. There are well over 200 species, of which ten can be claimed as British. Oddly enough, New Zealand, about the same size as Britain but with nearly five times as many molluscs, has but one member of the Family.

The Common Cockle, Cardium edule Linne, is one of the best known 'shellfish' in this country, and is consumed in large quantities in all parts of the British Isles, as it has been from time immemorial. It is the type species of Cerastoderma, a sub-genus of Cardium, of which Jeffreys writes, "This genus is redolent of the good old times. It carries with it a smack of true conservatism - progressive improvement without innovation. Every sound conchologist must rejoice at seeing the name Cardium preserved, with a few others, and to know that they have survived the extensive and often injudicious changes which systematiots have been continually propos¬ing since the death of the much-honoured Swede". Jeffreys may rest content that, after nearly a hundred years, the name Cardium is still with us, although many species once included in that genus now have other generic names, but are still included in the family Cardiidae.

Cockles are almost invariably sand dwellers, though a few species live in sandy mud. They bury themselves in the sand, using a large and powerful foot to do so. When the tide is in they often come to the surface of the sand and, using this foot, are able to propel themselves along in a series of hops, though the size of these hops has probably been exaggerated. In some cases it is more in the nature of a marine 'rock and roll'. In two of our own larger species, Cardium aculeatum and Cardium tuberculatum the foot is very large and bright red in colour, which has given rise to their Devonshire name of 'Red Nose'. Cockles are mainly sea dwellers, though some will live in estuaries. Cardium edule lamarcki will live in brackish water cut off from the sea, and two genera, Didnacna and Adacna live in fresh water in the Caspian Sea and the Danube.

Cockles are suspension feeders that is, they feed on plankton floating in the water. The two short syphons, which are united throughout most of their length, project just above the surface of the sand when the tide is in. Water enters the gill chamber through the lower syphon, is there strained through the meshwork of the gills, and leaves through the upper syphon. An interesting feature is the presence of pigment spots on the mantle and syphons, which may be sensory organs of sight, somewhat similar to the 'eyes' in the Pectinidae. These are not ej^es in the true sense of the word. They are merely sensitive to light, and should a shadow pass across, the animal immediately closes its shell.

While some species of Cardium have smooth shells, e.g. the British Cardiun (Laevioardium) erassum Gmelin, the majority are more or less strong¬ly ribbed. These radial ribs, with their intervening furrows, continue right to the edge of the shell so that the two valves interlock. Even the smooth forms have a sort of milled edges making a close fit. An unusual species is Cardium lyratum Sowerby, from Madagascar, in which half the ribs are radial, the other half almost concentric, giving the shell a most strange appearance. This interlocking of the valves is even more greatly emphasised in the Tridacnidae, which contains but two genera, Tridacna, which includes the Giant Clam of Australia, and Hippopus.

L. . Stratton