Limbs very slender; posterior nearly twice the length of the anterior.
3, Posterior end of alimentary canal.
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 median and posterior extension of the body of the sternum is a direct outgrowth of the latter, therefore FIG.
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.