REPORT OX ZOOLOGY. 201 



nogomdes,2Li\(\. Holetree {Phalangium andiAcarus, Linn,). In 

 the Families Naturelles his arrangement is nearly the same. 

 There is simply a change with respect to the order in which the 

 families stand, with the addition of some new ones amongst the 

 Trachean Arachnida. But in the Cours d' Entomo logic we find a 

 third ordei', termed Aporohranches, occupying a middle station 

 between the other two. This new group, which is characterized 

 by having gills without any external opening, Latreille intends 

 should include the Pyaiogonida. It has been already mentioned 

 that these anomalous animals, which seem to form the passage 

 from the Arachnida to the Crustacea, are considered by Ed- 

 wards as belonging to the class last mentioned. 



It may be stated that Mr. Kirby appears likewise to be of 

 opinion that the Pulmonary and Trachean Arachnida should 

 not be included in the same class*. 



Mr. MacLeay has, however, expressed himself differently. He 

 maintains " that the division of the organs of respiration and 

 circulation is not to be depended on in the classical arrangement 

 of the Annulosaf." 



This last opinion, which will probably in the end be generally 

 assented to, has been adopted by Duges in a valuable memoir on 

 the Acari, published during the present year;}:. In the intro- 

 duction to this memoir Duges has made some observations on 

 the relation which subsists between the Acari and the rest of 

 the Arachnida. He remarks that there is nothing in the external 

 structure of these animals at all corresponding to those differ- 

 ences in the respiratory and circulatory organs which some au- 

 thors have made the basis of their arrangement. He thinks that 

 the value of the characters derived from these organs has been 

 overrated ; and in proof of this, that it is only necessary to 

 observe the striking changes which such organs undergo (in the 

 case of the Batrachian Reptiles and aquatic insects) in the same 

 individual at different stages of its life. 



Instead, then, of making the external form subordinate to the 

 organs of respiration and circulation, M. Duges adopts the for- 

 mer as the groundwork upon which he establishes his principal 

 divisions. The following are what he considers as the true di- 

 stinguishing characters of the class Arachnida : Istly, the pre- 

 sence of eight feet adapted for walking ; 2ndly, the absence of 

 antennae§ and reticulated eyes ; 3rdly, the constant union of one 



• Introd. to Entom., vol. iii. p. 21. + Hor. Enf.,Tp. 382. 



J " Recherches sur I'Ordre des Acariens" Ann. des Scien. Nat. for Jan. 1834, 



§ Lamarck had observed, and formerly Latreille also, how strikingly the true 

 Arachnida were distinguished from the two classes of Crustacea and Insects by 



