Limbs very slender; posterior nearly twice the length of the anterior.
This so-called third, upper or posterior conch is not a true conch, nor is that of the vestibulum; only the middle one forms a scroll, and this corresponds to the only one of reptiles and the lower of the mammals.
The nervous system is thus essentially epidermal in position and diffuse in distribution; but an interesting concentration of nerve-cells and fibres has taken place in the collar-region, where a medullary tube, closed in from the outside, opens in front and behind by anterior and posterior neuropores.
Mark's narratives of the sepulture by Joseph of Arirathea and of the empty tomb are taken as posterior to St Paul; the narratives of the infancy in Matthew and Luke as later still.
Thus the postcaval vein of the higher vertebrata is partly a new structure altogether, and is partly formed out of the pre-existing posterior cardinals.