1909.] NATURAL SCIENCES OF PHILADELPHIA. 313 



and an unpaired or paired (Dysderida?) tracheal stigma; the excep- 

 tions are the Caponiida? with four tracheal stigmata, but no lung-books, 

 and the Pholcidae with one pair of lung-books, but with no trachea 1 

 or only very rudimentary ones (Lamy, 1902). 



There has been much discussion as to the genetic relations of these 

 two organs. They have been so fully and ably compared by Lamy 

 that it is not necessary to more than summarize his account. Since 

 the time of Leuckart (1849) all writers on the subject seem to conclude 

 an essential homodynamy of lung-books and tracheae, save Berteaux 

 (1889), who, however, reached quite an erroneous idea of the structure 

 of the pulmonary lamella?. But three different opinions have arisen, 

 namely, whether the lungs are modified trachea?, the tracheae modi- 

 fied lungs, or both derived from a common ancestral organ. That 

 the lungs are modified tracheae is held by Leuckart (1849), Ley- 

 dig (1855), Bertkau (1872). Schimkewitsch (1884, 1887), Croneberg 

 (1888), Schneider (1892), Sinclair (1892), v. Kennel (1892), Jaworow- 

 ski (1894), Lamy (1902), and Janeck (1909). That the tracheae are 

 derived from lung-books is maintained by Lankester (1881), MacLeod 

 (1884), Kingsley (1893), Simmons (1895), Laurie (1894), Wagner (1895) 

 and Kishinouye (1890) — this group of naturalists being mainly influ- 

 enced by the Limulus-ancestry theory. Finally, Weissenborn (1886) 

 and Bernard (1893) hold that lung-books and tracheae are divergent 

 derivatives of a common ancestral organ, in the one case from tracheae 

 of the type of Peripatus, in the other from acicular glands of Annelids. 



In their adult condition lungs and trachea? are both branched cavities 

 lined by cuticulated h}-podermis. In the lung-books the branches 

 are flattened lamellae placed parallel, in the tracheae the branches are 

 usually cylindrical ramifying tubes. But sometimes the main tracheal 

 trunk has no branches, and sometimes the branches are arranged in 

 parallel bundles, all of which details have been described by Lamy; 

 further, I have noticed in Loxosceles that the large median tracheal 

 trunk is excessively compressed, quite as much as a pulmonary lamella. 

 Further, trachea? and lung-books may replace each other in both the 

 second and third abdominal segments. Therefore there are no radical 

 anatomical differences between the two. 



Ontogenetically both arise as ectoblastic invaginations, and there is 

 a vestibulum in each case. But there are four differences in the develop- 

 ment: (1) In the case of the lung-books the abdominal appendage 

 invaginates and the wall of the part so invaginated bounds the main 

 pulmonary chamber, while the external surface of the appendage 

 becomes the operculum. In the case of the trachea the abdominal 



