452 ARTICULATED ANIJIALS. 



[It is to be observed, that tbese tvi'o orders are regarded by various celebrated 

 naturabsts as too widely distinguisbcd from eaeli other to remain in the same class. 

 Tliis idea was first entertained by Dr. Leaeb, (Zoolut/irfil Misc-t/iiin/, vol. iii. 1SI7), 

 wlio restricted the class to the families Seorjiionida', 'I'araiitidida', Plialaugida:, yolpu- 

 gid;c, and Araneida;, all (jf which were assumed to breatlu^ by means of pulmonary 

 saes, whilst the Trachearia of Latr. (excepting the Pyenogonid;e and Phalangida;), 

 were fijrrned into a separate class, which lie proj)osed to name Acari. Even Latreille 

 himself, in his Cours d'Eiitomologif, thought it necessary to separate the Pycnogonides 

 into a distinct order of the class Arachnida, ■which he named Aporobranchia. 

 Messrs. Kirby and Spence (Inirod. tu Entomology, vol. iii. p. 21) were als-o of opinion 

 that tlie Pulmonary and Traehean Ar.icbnida should not be included in the same class; 

 but Mr. MacLeajr (Hor^e Eittomoiogiae, p. 382) maintained that the diversity of the 

 organs of respiration and circulation is not to be depended upon in the classical arrange- 

 ment of the Annulosa ; and more recently Duges, in his memoir uiion the Acari, 

 adopted a similar view, considering that external form and general coincidence of 

 characters, sucli as the presence of eight feet for walking, the absence of organs used 

 as iintennae and reticulated eyes, and the constant union of the head and thorax, are of 

 more imiiortanee than the variations in the organs of respiration and circulation. I'his, 

 whicli I cvinsider as the most philosoiibical vie\v of the subject, (confirming as it docs 

 my observation on the distribution of the Crustacea proposed by M. Duverney, ante, 

 p. 410, note,) has been still more recently confirmed by Duges, wdio has read a memoir 

 before the French Institution, in wliich the genera Dysdera and Segcstria, belonging 

 to tlie Spiders, are stated to possess four spiracles, two of wdiich are connected with 

 pulmonary, a.nd two ■.vith traehean organs (see Giirrhi, Jiiill. Zuol.. No. '2). This 

 author has illustrated this structure in tlij t'rochard edition of the lilync Animal, 

 livr. It), Araclinides, pi. 10, f. 4. With tlie view of adapting the arrangement of Leach 

 to that of Latrcdle, I have proposed the following distribution of the class {Ent. Text 

 Look, p. 131). 



Section I. Pltlviox.msia, Latr. 



Order 1. Dimprosomatn, Leach, Araneio'es, Latreille. 



Order '2. Polymerosomalii, Leach, Pcd'qxilpl, Latrcdle, {Scorpionhhc and P/in/nidte). 

 Section II. TuAcuE.vui.v, Latr. 



Order 3. Adidnrtlirosomuta, Westw., couijiosed ot the tamilies So/pur/idiC, Cheli- 

 feruUe, and Phuhnnjiid^'. 



Order 4. Monomerosomata, Leach, restricted to the Acari. 

 Section III. Aporobrancri.v, Latr. 



Order 5. Podosomuta, Leach, consisting of the single family Pi/rnogoiiida. 



The Baron Walckcnaer, in his valualile Hisloirc No In relic des Liseclcs Aptlrcs, 

 (Paris, ly37, Svo, tom. i.), has divided the Araidniida of Latreille, wdiich he names 

 Aelrci-, after Lanuirck, (not ado[iting the views of Latreille that the cbellcera; are modi- 

 fiid antenna;), into six orders: — l.The Aranf.idios (T/ienijh'wses and Aruiij/ices) ; 

 2. PnuYNiiiDiiS {Pltrijnns, Tlielijplioinis) ; 3. SeourioNinivs [Scorpio, C/ielifer, and 

 Ooisiiim) ; 4. SoLi'U'jiDES (Ga/codcs) ; 5. Phalanguiks (PliulnnijiiDn, Siro, iShicro- 

 c/ieles, Trnijnliif:, and " Miti:s") ; G. yVe.AUinEs (TromhiiliiDn , j 1 ijilriieliini , (,'iuiiii;ni::, 

 Ixodes, Aenras, Jiii/hiit;, Bdella, and Orilnilii). Thus we find that the rcspiratoi'y organs 



