ST[IDIES ON TllR ARACHNID KNTOSTKKNITE. 257 



cord, to say notliing of the large tracheal apertures on each 

 side. Hence if the diaphragm originated from an integu- 

 mental infohiing it has secondarily encircled the three median 

 organs above mentioned, and is a highly specialised structure. 

 But, as a matter of fact, in the "diaphragm" of Claleodes 

 I can find no evidence of such a derivation. It appears to 

 be formed of muscular and connective tissue like that of the 

 Scorpions, and to have had an internal origin quite apart 

 from the integument. The infolding of the integument 

 Bernard speaks of appears to be quite superficial, and to 

 occur only at the periphery of the diaphragm. 



Whether this diaphragm has been developed independently 

 of the diaphragm of Scorpions, to which it is similar in its 

 structure and position, or whether the two are to be regarded 

 as a heritage from a common ancestor, are matters of quite 

 another kind. The absence of such an organ in the Pedi- 

 palpi, Aranete, Pseudo-scorpiones, Opiliones, and Acari, 

 coupled with the wide structural divergences between the 

 Scorpions and Solifugje, points to the independence of the 

 origin of the diaphragm in these two orders in response to 

 similar physiological needs. 



Note. — I have elsewhere suggested (see ^ Ann. Mag. Nat. 

 Hist.,' 1893) that the value of the structural characters of the 

 ordei's of terrestrial Ai^achnida may be expressed by grouping 

 them into four divisions of superordiual rank : the first to 

 contain the Scorpions ; the second the Pedipalpi and the 

 Araneae; the third the Solifugaj ; the fourth the Pseudo- 

 scorpiones, Opiliones, and Acari. A study of the entosternites 

 confirms this classification in a remarkable and unexpected 

 degree, especially as regards the isolation of the Scorpiones 

 and Solifugjfi, and the association of the Aranefe with the 

 Pedipalpi. 



Bibliography. 



1. Beck, E. J. — " Description of the Muscular and Endoskeletal Systems of 

 Scorpio," ' Trans. Zool. Soc. London,' xi, 1SS5. 

 VOL. 46, PART 2. — NEW SERIES. R 



