452 Div. 3. ARTICULATA.-^ARACHNIDA. Class 2. 



[It is to be observed, that these two orders are regarded by various celebrated 

 naturalists as too widely distinguished from each other to remain in the same class. 

 This idea was first entertained by Dr. Leach, {Zoological Miscellany, vol. iii. 1817), 

 who restricted the class to the families Scorpionidae, Tarantulidse, Phalangidae, Solpu- 

 gidse, and Araneidas, all of which were assumed to breathe by means of pulmonary 

 sacs, whilst the Trachearia of Latr. (excepting the Pycnogonidae and Phalangidse), 

 were formed into a separate class, which he proposed to name Acari. Even Latreille 

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

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

 Messrs. Kirby and Spence {Introd. to Entomology , vol. iii. p. 21) were also of opinion 

 that the Pulmonary and Trachean Arachnida should not be included in the same class ; 

 but Mr. MacLeay (Hora Entomologicce, 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 Dug&s, in his memoir upon the Acari, 

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

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

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

 more importance than the variations in the organs of respiration and circulation. This, 

 which I consider as the most philosophical view of the subject, (confirming as it does 

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

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

 before the French Institution, in which the genera Dysdera and Segestria, belonging 

 to the Spiders, are stated to possess four spiracles, two of which are connected with 

 pulmonary, and two with trachean organs (see Gu^rin, Bull. ZooL. No. 2). This 

 author has illustrated this structure in the Crochard edition of the Regne Animal, 

 livr. 10, Arachnides, pi. 10, f. 4. With the view of adapting the arrangement of Leach 

 to that of Latreille, I have proposed the following distribution of the class (Ent. Text 

 Book, p. 131). 



Section I. Pulmonaria, Latr. 



Order I. Dimerosomata, Leach, Araneides, Latreille. 



Order 2. Polymerosomata, Leach, Pedipalpi, LatreUle, {Scorpionidce and Phrynidxe). 



Section II. Trachearia, Latr. 



Order 3. Adelarthrosomata, Westw., composed of the families Solpugidce, Cheli- 

 feridcE, and Phalangiidte. 



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



Section III. Aporobranchia, Latr. 

 Order 5. Podosomata, Leach, consisting of the single family Pycnogonidce. 



The Baron Walckenaer, in his valuable Histoire Naturelle des Insectes Aptires, 

 (Paris, 1837, 8vo, tom. i.), has divided the Arachnida of Latreille, which he names 

 Aclres, after Lamarck, (not adopting the views of Latreille that the chelicerse are modi- 

 fied antennae), into six orders: — 1. The Araneides {Theraphoses and Araign^es) ; 

 2. Phryneides {Phrynus, Thelyphonus) ; 3. Scorpioniues (Scorpio, Chelifer, and 

 Obisium) ; 4. Solpugides (Galeodes) ; 5. Phalangides (Phalangium, Siro, Macro- 

 cheles, Trogulus, and "Mites"); 6. Acauides (Tromhidium, Hydrachna, Gamasus, 

 Ixodes, Acarus, Eylais, Bdella, and Oribata). Thus we find that the respiratory organs 



