C antero-posterior direction, its ventral lobe usually concealing the mouth.
Of the muscles of the stem or axis, those of the neck and tail are well-developed and specialized, while those of the lower back are more or less reduced, or even completely degenerated owing to the rigidity of this region, brought about by the great antero-posterior extent of the pelvis.
The first is a ventral flexure in the antero-posterior or sagittal plane; the result of this is to approximate the two ends of the alimentary canal.
(Lankester.) x, y, The median antero-posterior axis.
K antero-postero median section 6, 7, with following additions.
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