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The following features seem to be common to most of the Acari, 
and they are not yet found in other Arachnids, viz: that they have 
a three-legged larval form, and that there is an inert stage before 
each ecdysis (Michael I p. 195). These two points of similarity 
are certainly very important, but it-seems to me that they are not 
sufficient: for a quite sure foundation of an order. Å number of 
other characters are common in a higher or smaller degree to the 
different members, but from different causes they are not important 
enough to settle the question. 
1) There are points of similarity in the shape of the mouth 
organs.' The maxillae are fused with each other and sometimes with 
the labrum; we have often.a movable pseudocapitulum, forméd by 
the fusing of .the maxillae with the rostrum. Only the first of 
these characters is common to all Acari, but it is also found in 
the Pédipalpi and the Cryptostemma. But as the mouth organs of 
the Qamasidae and JIxodidae are just as different from each other 
as those of the Gamasidaée and Pedipalpi (comp. C. Bårner 4), 
they do not prove the systematic unity of the Acari. 
2) With regard to the chela of the antennae and other ap- 
pendages Oudemans (17 pag. 46) ,,fand dass, wenn .... Acarina 
Scheeren haben, der digitus mobilis innen oder oben liegt..." and 
resumed his results as: follows (p. 45—46) ,,Ich habe ......- ge- 
funden, dass der digitus mobilis bei allen Arachnoidea und bei 
Limulus aussen oder unten, bei allen Crustacea dagegen innen oder 
oben liegt, ganz gleichgiltig von welchen Extremitåt man die Scheere 
nimmt". But many ÅAcari, f. inst. Gamasidae and Oribatidae, have 
a chela with a lower movable finger, and as thus the base for 
Oudemaåns' theory is wrong, 10 further discussion of this matter 
seems necessary (comp. Wagner 19 pag. 148). 
3) Most authors agree in the theory that the body of the 
Acari falls in two distinet parts, separated by a groove behind the 
second pair of legs, and they are certainly right; the absence of this 
groove in many full-grown Acari (most Gamasidae and Oribatidae) 
does not weigh much. But the theory of Dr. Sørensen, involving 
