LUNG-BOOK OF SCORPIO AND GILL-BOOK OF LIMULUS. 339 
A New Hypothesis as to the Relationship of 
the Lung-book of Scorpio to the Gill-book 
of Limulus. 
By 
E. Ray Lankester, M.A., L.L.D., F.R.S., 
Jodrell Professor of Zoology in University College, London. 
The view which I advocated in my essay f Limulus an 
Arachnid/ as to the mode of conversion of an external lamel- 
ligerous appendage into the hollow lamelliginous lung of 
Scorpio no longer commends itself to me. A much simpler 
and, as it appears to me, a thoroughly satisfactory explanation 
of the relationship of the two organs, has occurred tome in the 
course of recent investigations, and is supported also by em- 
bryological data. In the essay above referred to I suggested 
that by the enlargement of the hollow stigmata connected 
with the thoraco-branchial muscles of an ancestral Scorpion, 
resembling Limulus in having branchigerous appendages on 
the mesosoma, and provided with thoraco-branchial muscles, 
the branchigerous appendage might come to lie in the pit or 
hollow of the tendon of the thoraco-branchial muscle, and 
eventually the hollow might enclose it. The conversion of the 
insunken appendage into a hollow air-holding sac and the 
corresponding conversion of the surrounding pit into a closed 
blood-holding space, involved serious difficulties which were 
indeed fatal to the hypothesis. 
When I found ( f Transact. Zool. Soc./ vol. xi, part x, 1884) 
that the muscle (veno-pericardiac) attached to the apex of each 
lung-sinus in Scorpio had no possible relation to the thoraco- 
branchial muscles of Limulus, but was represented in 
Limulus by exactly similar veno-pericardiac muscles, I gave 
