290 MR. H. M. BEENARD ON THE EELA.TIO]Sr 



on the second abdominal segment of the Araneids. Pagen- 

 stecher's original drawing of the tracheae opening through these 

 stigmata is very instructive. We have a short thick trunk, 

 ending in a close tuft of short tubules. They lie, not only in 

 position on the body, but also in structure, halfway between the 

 book-leaf tracheae and the purely tubular thoracic tracheae. Now, 

 while it is impossible to assume that the thoracic tubular tracheae 

 came from embedded gills, it is not only not impossible, but 

 highly probable, that both tubular and book-leaf tracheae were 

 derived from such tuft-like tracheae opening as we have described 

 on the first abdominal (or ? last thoracic) segment of Ixodes. 

 The tubular tracheae are tuft-tracheae specialized for tlie respi- 

 ration of the tissues directly. The book-leaf tracheae are spe- 

 cialized for the oxygenation of a bloodstream, and thus of the 

 tissues indirectly. A close parallel to this variation of tracheal 

 arrangement in the Arachnids is afforded by the Myriapods, 

 where, though tubular tracheae are the rule, one group, the Scuti- 

 geridae, have a kind of book-leaf system specialized for the 

 aeration of the blood in the pericardium *. 



If, therefore, the Acaridae are true Arachnids, their tracheae 

 seriously complicate the position of those w^ho would sever the 

 Arachnida from the other Tracheata. Instead of one origin for 

 these remarkable breathing-organs, they are forced, as above 

 described, to assume four more or less distinct origins. 



Although the important bearing of the Acaridae on this 

 question of the origin of the Arachnida has by no means been 

 overlooked t, it has not received the attention it deserves. To 

 me it appears strongly to confirm the old classification which 

 placed the Arachnida with the other Tracheata. 



Special Literature. 

 Claparede. — "StudienanAcariden."Zeitschr.wiss. Zool.1868. 

 DoNNADiEU. — Ann. Soc. Linneenne de Lyon, tome xxii. 

 Halleb. — " Die Mundtheile und systematische Stellung der 



Milben." Zool. Anzeiger, 1881. 

 Henking. — "Anatomie, Entwickelung und Biologic d. Trom- 



hidiuni fidiginosumy Zeitschr. wiss. Zool. 1882. 



* Sinclair, "A new Mode of Kespiration in the Myriapoda," Ann. &Mag. N. 

 H., March 1892. 



t Weissenborn, " Beitrage zur Phylogenie der Arachnideu," Jenaische Zeitsch. 

 Bd. XX., 1885. 



