COCKLE, in zoology, a mollusc (Cardium) of the class Lamellibranchia (q.v.). A very large number of species of Cardium have been distinguished by conchologists. Besides the common species Cardium edule, two others occur in Britain, but are not sufficiently common to be of commercial importance. One of these is C. echinatum, which is larger than the common species, reaching 3 in. in diameter, and distinguished by the presence of spines along the ribs of the shell. The other is C. norvegicum, which is also somewhat larger than C. edule, is longer dorso-ventrally than broad, and is only faintly ribbed.
The two valves of the shell of the common cockle are similar to each other, and somewhat circular in outline. The beak or umbo of each valve is prominent and rounded, and a number of sharp ridges and furrows radiate from the apex to the free edge of the shell, which is crenated. The ligament is external, and the hinge carries cardinal teeth in each valve. The interior of the shell is remarkable for the absence of pearly lustre on its interior surface. The colour externally is reddish or yellowish. The pallial line, which is the line of attachment of the mantle parallel to the edge of the shell, is not indented by a sinus at the posterior end. In the entire animal the posterior end projects slightly more than the anterior from the region of the umbones.
The animal possesses two nearly equal adductor muscles. The edges of the mantle are united posteriorly except at the anal and branchial apertures, which are placed at the ends of two very short siphons or tubular prolongations of the mantle; the siphons bear a number of short tentacles, and many of these are furnished with eye-spots. The foot is very large and powerful; it can be protruded from the anterior aperture between the mantle edges, and its outer part is bent sharply forwards and terminates in a point. By means of this muscular foot the cockle burrows rapidly in the muddy sand of the sea-shore, and it can also when it is not buried perform considerable leaps by suddenly bending the foot. The foot has a byssus gland on its posterior surface.
On either side of the body between the mantle and the foot are two flat gills each composed of two lamellae. Cardium belongs to the order of Lamellibranchia in which the gills present the maximum of complexity, the original vertical filaments of which they are composed being united by interfilamentar and interlamellar junctions. In other respects the anatomy of the cockle presents no important differences from that of a typical Lamellibranch. The sexes are distinct, and the generative opening is on the side of the body above the edge of the inner lamella of the inner gill. The eggs are minute, and pass out into the sea-water through the dorsal or exhalent siphon. The breeding season is April, May and June. The larva for a time swims freely in the sea-water, having a circlet of cilia round the body in front of the mouth, forming the velum. The shell is developed on the dorsal surface behind the velum, the foot on the opposite or ventral surface behind the mouth. After a few days, when the mantle bearing the shell valves has developed so much as to enclose the whole body, the young cockle sinks to the bottom and commences to follow the habits of the adult. The usual size of the cockle in its shell is from 1 to 2 in. in breadth.
The common cockle is regularly used as food by the poorer classes. It occurs in abundance on sandy shores in all estuaries. At the mouth of the Thames the gathering of cockles forms a considerable industry, especially at Leigh. On the coast of Lancashire also the fishery, if it may be so called, is of considerable importance. The cockles are gathered by the simple process of raking them from the sand, and they are usually boiled and extracted from their shells before being sent to market. The cockle is liable to the same suspicion as the oyster of conveying the contamination of typhoid fever where the shores are polluted, but as it is boiled before being eaten it is probably less dangerous. (J. T. C.)