The process of burrowing in the Bivalvia has been studied in a restricted range of species only, notably in members of the Solenacea (Fraenkel, 1927), the Lucinacea (Allen, 1958), and the Veneracea (Quayle, 1949; Ansell, 1962). This previous work, summarized by Morton (1964), makes little reference to burrowing in the common British littoral species and, with the exception of Ansell's paper, lacks the precision which more modern techniques of recording may give. The object of this research was to study the process of burrowing in detail in TelUna tennis, Macoma balthica, Donax vittatus and Cardium edule, common littoral bivalves with very different shell shapes and habits. Burrowing by a bivalve consists of successive cycles of activity during the penetration of the animal into the substrate. It appears that the general picture for Ensis holds good for other 'normal' bivalves, e.g. the Veneridae, and may be summarized as consisting of the following main stages which occur successively (Fraenkel, 1927): (a) Protrusion of foot into substrate until fully extended (Hakenform). (b) Dilation of the distal end to form an anchor (Schtcellform). (c) Pull downwards by contraction of pedal retractor muscles (Grabstufe). Fraenkel groups stages (a) and (b) together as the Grabschritt. This summary may be used as a basis for the comparison of the four species investigated. The events which occur in association with each downward movement will be referred to as the digging cycle equivalent to the term digging sequence of Ansell (1962), the term digging cycle, being preferred since the events comprising it are of a cyclical nature. The term digging period (or burrowing period) is used here to describe activity from the start of burrowing until the final position is reached, and generally consists of a large number of digging cycles.
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