—In the comparison of the lung books of Scorpio and the gill books of Limulus, given in a preceding portion of this memoir, and in the attempt to derive the two modifications of lamelligerous appendage from a common ancestral form, I have carried the supposed history only so far as to reach a hypothetical Scorpion-like form in which the lamelligerous appendage is supposed to be filled with blood, the “pulmonary sac,” or “investing sac,” or “cave of invagination “(the homologue of the funnel-like cavity of the tendon of the thoraco-branchials of Limulus), being still filled with air and communicating persistently with the atmosphere by means of a stigma, which in this case is the original orifice of invagination of the investing sac.
Such was probably the condition of an ancestral Scorpion. Tn living Scorpions a further development has taken place. The original stigma has become entirely closed up;...
