coiling In many univalve and bivalve molluscs (Mollusca) the shells are coiled. The condition is most noticeable among gastropods (Gastropoda) and cephalopods (Cephalopoda), where it is obvious that the shell is a hollow cone, coiled up to a greater or lesser extent. These rolled-up cones grow at the apertural end only and form a logarithmic spiral. Since the shell is a hollow cone, coiling about a vertical axis and growing at the apertural end, it is possible to generate a number of shapes. The shape of the tube in section (known as the ‘generating curve’) when expanding and coiling around and away from the vertical axis in a single plane defines a ‘planispirally coiled’ shell. If the coiling does not remain in a single plane but moves down the axis (translation), a helically coiled shell results. Where the translation down the axis is in a clockwise direction the coiling is termed ‘dextral’; where it is anticlockwise it is termed ‘sinistral’. In most cases the coils (whorls) remain in contact during expansion and coiling, but in some cases they do not, resulting in a loosely coiled or ‘disjunct’ shell.
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