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KINGSLEY. 



[Vol. VIII. 



distance and then an inpushing takes place just behind the 

 appendage, the opening of the invagination, according to Kish- 

 inouye being away from the median Hne. This sac, the future 

 puhnonary cavity, continues to increase in size, while the ap- 

 pendage proper soon becomes obsolete. In this way the pul- 

 monary sac, with its external opening or stigma, is developed. 

 After the pulmonary sac is formed there begins on its anterior 



wall, i.e. on the continuation of the posterior wall of the 



appendage, — a series of foldings of the ectoderm, the lung- 

 leaves. As the animal increases in size, new lung-leaves are 

 added at the inner or proximal end. In short, as the adjacent 

 diagrams show, the homologies between the two types of organs 

 are perfect. 



In / we have a condition which will apply equally well to 

 the young of either Limulus or Scorpio. At the right side 

 the appendage is just budded out, and on the left the sinking 



Diagram of the respiratory organs in {A) an Arachnid ; (Z) in Limulus, and 

 (/) in an intermediate condition. The arrows point toward the head, the cross- 

 lined portion is the anterior, the black, the posterior surface of the appendage, the 

 dotted surface that part of the ventral surface of the somite, which is invaginated 

 to form the posterior wall and roof of the pulmonary sac. bs, blood space ; 

 gl, gill-leaves ; //, pulmonary leaves. 



