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.
Subjects: Earth Sciences and Geography.