2 



8 KINGSLE Y. [Vol. VIII. 



Limulus and the Arachnids were it not that the question is 

 frequently misunderstood. The difficulty, at least in some 

 instances, seems to lie in the failure to recognize the possi- 

 bility of the tracheae of the Hexapods and those of the Arach- 

 nids being homoplastic rather than homologous organs. Thus 

 to these persons the attempt to derive the tracheae of the 

 Arachnid from the gill of some branchiate form seems to 

 imply the promotion of the Arachnids to the position of 

 the "Stammform" of the Tracheata, a conclusion which no 

 one would care to defend at the present day. 



There exists at present no question of the accuracy of the 

 view of Leuckart ('49) that the lungs of the scorpions and 

 spiders on the one hand and the tracheae of the Araneina 

 and other Arachnids are homologous, but these organs differ 

 in one important respect from the tracheae of the Hexapods 

 which would prevent their close comparison. In the Hexapods 

 (as also in the Chilopodous Myriapoda) the stigmata are 

 placed outside or dorsal to the appendages and they never 

 develop in connection with the legs. The observations of 

 Chun ('75) followed by those of later writers would tend to 

 show that the Hexapod tracheae were derived from dermaP 

 glands. In the Arachnids, observations are as yet lacking as 

 to the ontogeny of the tracheae, but several students have 

 described the development of their homologues, the pulmonary 

 organs. 



The lungs of scorpions — Metschnikoff, ('7i); Kowalevsky 

 and Schulgin ('86) Laurie ('90) — and those of spiders — Bruce 

 ('87); Locy ('86) — develop in connection with the abdominal 

 feet in the embryo. The lung-leaves arise as outgrowths upon 

 the posterior faces of these appendages, concomitantly with the 

 formation of a pit behind the appendage and the sinking of the 

 appendage itself. In this it is, making allowance for the position 

 of the organ — freely projecting in the one, sunken in the other 

 — closely comparable to the gills of Limulus, and in the later 

 stages, the resemblance extends to the minute histological details. 



1 The recent speculations of Bernard ('92, '93) in which the endeavor is made 

 to trace all tracheae — Hexapod and Arachnid — to the setiparous glands of the 

 Chaetopods should possibly be referred to here. 



