CLASSIFICATION OP ACAEINA. 31 



The main reason why I am inclined to prefer this to 

 Canestrini's classification are : (1) that it does not rely 

 so entirely upon a single system of organs (the respira- 

 tory) for its principal divisions, but endeavours to take 

 a wider basis ; (2) that the mites are treated as a sub- 

 class, not a class ; (3) that the question of terrestrial or 

 aquatic habitat is not insisted on as differentiating so 

 high a group. 



Among the points upon which I am not able to agree 

 with Dr. Trouessart are the following : 



It does not appear to me to be correct to say that in 

 the great majority of his Order I the abdomen is con- 

 founded with the cephalothorax ; it undoubtedly is so 

 in most of the Hydrachnidse and Sarcoptinae (I am 

 using these expressions in Trouessart's sense) ; but if 

 we are to understand the word abdomen in the sense 

 in which it is usually employed in reference to the 

 mites, and that sense seems to me correct, then there is 

 a very decided and well-marked division between 

 cephalothorax and abdomen in such groups as the 

 Trombidinaa, Cheyletidaa, Scirinaa, Tetranycinse, Bdel- 

 lidae, Gamasinae, Oribatidaa, Tyroglyphinae, and, indeed, 

 the great majority of the Acari. Trouessart evidently 

 felt this difficulty himself, because when he came to the 

 fuller definition of his order Acarina he says, " abdo- 

 men generally anchylosed to the cephalothorax and 

 more or less confounded with it " (the italics are mine), 

 and in the fuller definition of the Trombidieae, which 

 form Section b of his Trombidinas, and therefore an im- 

 portant division of his group Acarina, he says, "Cepha- 

 lothorax very distinct from the abdomen." Trouessart 

 also correctly defines his Hoplophorinaa, another sub- 

 family of his Acarina, as " Cephalothorax moveable 

 upon the abdomen." Of course, in the views of those 

 biologists who consider that in the Arachnida nothing 

 can be considered an abdomen which is provided with 

 legs, the vast majority of the Acari cannot be considered 

 to have any abdomen at all ; amongst these is Dr. A. C. 



