LIMULUS AN ARACHNID. 609 
Limu us an ARACHIND. By E. Ray Lanxester, M.A., F.R.S., 
Jodrell Professor of Zoology in University College, London. 
(Continued from p. 548.) 
Further remarks on the pulmonary sacs and lung books 
of Scorpio.—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 modi- 
fications of lamelligerous appendage from a common an- 
cestral 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 in- 
vagination ” (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. 
In living Scorpions a further development has taken place. 
The original stigma has become entirely closed up; the in- 
vesting sac—that which I have spoken of as pulmonary sac 
—contains no longer air but blood. A mew opening (the stig- 
matic slit) has formed within the area formed by the ciosure of 
the primitive opening of the cave of invagination in the form 
of a slit-like fissure in the delicate membranous wall of the axis 
of the in-sunken pulmonary appendage (see woodcut, fig. 15). 
By this aperture air now enters where, in Limulus and the 
J 
was 
ee 
Pune) aay 
Fig.15 
Fic. 15.—Diagram of a Scorpion’s lung-book, enclosed in the pulmonary 
sac and divided by a cut at right angles to the lamelligcrous axis, 
ax, axis; 1, lamella; ps, pulmonary sac or cave of invagination; m, 
raised margin of the stigma ; s/, slit leading from the exterior into the 
axis of the lamelligerous appendage. 
