0. L. Simmons — Development of Lungs of Spiders. 121 



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 inpushings of 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 con- 

 ditions would carry the appendage within the body wall where 

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

 posterior end of the lung cavity and the lamellae projecting 

 toward the posterior end of the body. As Kingsley states, 

 viewed from a histological standpoint, the description of the 

 pulmonary organ of the spider or scorpion applies, almost word 

 for word, to the gill-book of Limulus. He believes that the 

 lungs of spiders are the primitive and the tracheae the derived 

 structures. The tracheae of the Hexapoda have no relation to 

 the tracheae of spiders, having an entirely different origin. 



Malcolm Laurie ('90) in his article on " The Embryology of 

 a Scorpion " thinks the lung-books are undoubtedly compara- 

 ble to the abdominal appendages of Limulus but hesitates to 

 decide which of two propositions he advances is the correct 

 one. He inclines toward the view that the lung-books of 

 Scorpions are invaginated, i. e. the edge of each lamella in the 

 Limulus gill-book corresponds to the interior fold between the 

 lamellae in the Scorpion lung-book. He imagines that he sees 

 difficulties in explaining his second proposition which states 

 that the whole appendage has sunk without invagination into a 

 cavity in the abdominal surface. By either proposition the 

 air space of the primitive condition would be air space in the 

 derived condition. 



On the other hand many comparative anatomists, recogniz- 

 ing the homologies pointed out years ago by Leuckart, as exist- 

 ing between the lungs and tracheae of spiders and believing 

 that these last were the homologues of the structures known 

 by the same name in the Hexapods, have failed to recognize 

 the cogency of the reasoning of the advocates of the Arach- 

 nidan affinities of Limulus. Thus Arnold Lang in his Lehr- 

 buch der vergleichenden Anatomic (II Heft, p. 548, 1890) 

 writes that the respiratory organs of Arachnoidea are tracheae 

 — tubular and book-leaf tracheae. His view of the morpho- 

 logical signification of the latter is that they are modified 

 tracheal tufts which, standing close together, have been flat- 

 tened into hollow plates. He believes that the view of those 

 who would bring the gill-books of Limulus and the lung-books 

 of scorpions and similar types into close relationship, is arti- 

 ficial and unsupported by comparative anatomy and ontogeny. 



So, too, Bernard ('92) in " The Apodidae" says it is easiest 

 to believe that the lung-books of the various forms are only a 



