172 
that this distinction between the ,,4ead" and the ,,thorax" is a 
common aårachnological character, makes it impossible to use this 
fact in the definition of the Acari. 
4) One of the most stricking features in the Acari is ,,Ab- 
domen fused with cephalothorax and body unsegmented". After a 
long discussion about this theme Michael says (12 p. 21) ,,The 
fact seems to be that the proper definition of the Acarina (treating 
that word as including all the Mites) would be ,,Abdomen fused 
with the cephalothorax, or united to it by almost the whole breadth"; 
but if that be adopted, what becomes of the book definitions of the 
difference between Acari and Phalangiidae?" And not only Pha- 
langiidae but also other Arachnids (f. inst. Chelonethi) especially 
when we regard such a segmented form as Eucarus. 
5) The absence of tracheae, heart, coxal glands and endosternite 
in many Acari has scarcely any systematic importance in giving a 
characteristic of all Mites. The concentration of the nervous mass 
is also found in the Araneae and Chelonethi. 
According to Wagner the two last named groups of'facts are 
only a consequence of their degeneration or more correctly of their 
degradation from higher Arachnids. Other systems of organs differ 
in a surprising degree in the Acari, f. inst. the breathing, excretory 
and .digestive organs; this circumstance is more suitable for sepa- 
rating than for uniting the different Acari. 
The question before us, whether the different suborders form 
a natural group or not, will probably first get its definitive solution, 
a universal comparaison between the different forms based on all 
features of systematic importance being undertaken. 
If these doubts are founded, it must consistently be the wrong 
way to compare the Acari as a whole with other Arachnids or 
Arthropods; the different authors have also arrived at very different 
results from this base. Haller's (7) and Oudeman's (17) 
theory that the Acari are not Arachnids has been refuted by 
Wagner. (19) who — and probably rightly — like most other 
naturalists regards the Acari as degraded Arachnids. The Finnish 
