212 Mr. 0. L. Simmons on the 



were lamelligerous. These latter in Scorpio became smaller 

 and served only for respiratory organs, soon becoming air- 

 breathing. The four hinder pairs took a "trick" of growth, 

 viz. an invagination of the appendages, beginning at their 

 distal ends, so that they grew into the scorpion's body, turning 

 their outside in, just as a glove may be turned wrongside out, 

 beginning at the ends of the fingers. Thus the appendages 

 would be tucked into the blood-sinus instead of growing out 

 normally. The blood-sinus would become a venous sac 

 around the appendage. He explains the " trick " of growth 

 by the least-resistance theory — the pressure being exerted on 

 the embryo before it leaves the mother. 



J. MacLeod (1882 and 1884) sets forth an hypothesis by 

 which he develops Scorpions and similar forms from a 

 Limulus-like ancestor. His first proposition is that the 

 abdomen of Limulus be considerably elongated without other 

 change. This would cause the imbrication of the members to 

 cease — each appendage would stand out by itself although 

 closely following each other. Then suppose that the sternal 

 plate increase in size and unite with the ventral surface of 

 the abdomen. Thus the gill-book cavity would be entirely 

 filled up by the sternal plate except in those cavities on the 

 ventral side which contain the gills or lungs, now greatly 

 reduced in size. In this condition the lungs are quadrangular 

 ]dates, attached by two edges only. Inserted on each plate 

 is a number of lamellaj which are attached by one side only. 

 In this condition when removed from the water the lamellge 

 would cling together and be imperfectly in contact with the 

 air. To be of service the lamellse would have to unite their 

 lateral edges to the plate, leaving only the posterior edge free. 

 I'lius MacLeod developed the lamellae and other structures of 

 the lungs of Arachnida. By a comparison of structures in 

 the adult form MacLeod came to the conclusion that the 

 tracliefe of Spiders are developed from the lungs. 



J. S. Kingsley (1885 and 1893) advances a much simpler 

 exjjlanation to account for the transition from a Limulus gill- 

 book to the lung-book of a spider or scorpion. By a sinking- 

 in of the whole appendage bearing the gill-leaves and an 

 increase of the inpushingsof the integument and a decrease in 

 the outgrowths the whole matter is explained. This involves 

 a diminution of growth on the anterior side of the appendage 

 and an increase of growth on the posterior side. These 

 conditions would carry the appendage within the body-wall, 

 where it would be situated as seen in the adult — the spiracle 

 at the ])CSterior end of the lung-cavity and the lamellae pro- 

 jecting toward the posterior end of the body. As Kingsley 



