The axis of orientation of the synlophe in the Heligmosomoidea (Nematoda, Trichostrongylina): a new approach.

The definition of the axis of orientation of the synlophe is modified for the Heligmosomoidea so that one or two axes may be recognized. When two axes are present, their inclinations to the sagittal axis are different on the right and left sides, and we propose to name them right axis and left axis, respectively. During the course of evolution, starting from a single oblique axis (plesiomorphic state), an independent rotation of this axis on the right and left sides may bring about a double-axis state with a different inclination on both sides (derived state). When the rotation reaches 90 degrees for both sides, the axis becomes simple once again and is superimposed to the frontal axis (most derived state).