120 0. L. Simmons — Development of Lungs of Spiders. 



been converted into a blood space. The atropy of the muscles 

 which move the gills in Limulus and similar forms was con- 

 sidered very essential to this theory. The difficulties involved 

 in the changes of blood and air spaces were so considerable as 

 to prevent the acceptance of this hypothesis. 



Later, Lankester ('85) put forth a new theory. Because of 

 discoveries concerning- the muscles (veno pericardiac) of 

 Scorpio, as well as on account of the insuperable difficulties of 

 his previous view, he gave up his old and advanced a new 

 hypothesis. In the latter the common ancestor is assumed to 

 have had six pairs of mesosomatic appendages, of which five 

 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, turn- 

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

 out, beginning at the ends of the fingers. Thus the append- 

 ages 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 ('82 and '84) 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 append- 

 age 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 con- 

 tain the gills or lungs, now greatly reduced in size. In this 

 condition the lungs are quadrangular plates, attached by two 

 edges only. Inserted on each plate is a number of lamellae 

 which are attached by one side only. In this condition when 

 removed from the water, the lamellae would cling together and 

 be imperfectly in contact with the air. To be of service the 

 lamellae would have to unite their lateral edges to the plate, 

 leaving only the posterior edge free. Thus MacLeod devel- 

 oped 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 tracheae of Spiders are devel- 

 oped from the lungs. 



J. S. Kingsley ('85 and '93) advances a much simpler, ex- 

 planation to account for the transition from a Limulus gill- 



