Development of the Lungs of Spiders. 213 



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 (1890), in his article on " The Embryology 

 of a Scorpion," thinks the lung-books are undoubtedly com- 

 parable to the abdominal appendages of Limulus , but hesitates 

 to decide which of two propositions he advances is the correct 

 one. He inclines towards 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 invagina- 

 tion into a cavity in the abdominal surface. By either propo- 

 sition the air-space of the primitive condition would be air- 

 space in the derived condition. 



On the other hand, many comparative anatomists, recog- 

 nizing the homologies pointed out years ago by Leuckart as 

 existing 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 

 Arachniclan affinities of Limulus. Thus Arnold Lang, in his 

 ' Lehrbuch der vergleichenden Anatomie ' (ii. Heft, p. 548, 

 1890), writes that the respiratory organs of Arachnoidea are 

 tracheae — tubular and book-like tracheae. His view of the 

 morphological signification of the latter is that they are 

 modified tracheal tufts which, standing close together, have 

 been flattened 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 relation- 

 ship is artificial and unsupported by comparative anatomy 

 and ontogeny. 



So, too, Bernard (1892), in { The Apodidae,' says it is 

 easiest to believe that the lung-books of the various forms are 

 only a specially concentrated arrangement of tracheal tubes. 

 He regards the tracheae, including the lungs of all " Tra- 

 cheates," as having their origin in dermal glands which have 

 gradually been modified for respiratory purposes. He also 

 states that in considering the relationships of these various 

 forms the limbs are of so little importance that one might 

 almost be tempted to leave them out of account. In a later 



