21 
The President's Address. By A. D. Michael. 
and the whole arrangement of the internal organs and of the muscu- 
lature seems to favour that view. If we turn to the writings of the 
specialists in acarology, we find that as a rule they shirk this difficult 
question, starting their classification from the base-point of Acarina, 
which group they presume to be sufficiently marked out by the 
general zoologist. Some, however, have not done this; but they, 
although they mostly do not wholly discard the fusion of cephalo- 
thorax and abdomen as a means of identifying the mites, treat it in a 
very uncertain manner ; and when they come to define the families 
they usually find themselves compelled to admit the existence of a 
separate abdomen. Take as an example the classification of Trouessart 
of Paris, which is the latest and, on the whole, the best : he makes 
the Mites a sub-class — Acaroidea — of which he says, “ Abdomen 
broadly joined to the thorax and generally fused with it.” He 
divides his sub-class into two orders, which he calls Acarina and 
Vermiformia. The latter he defines as “ abdomen distinct from the 
cephalothorax ” ; the former, in his table, as “ abdomen anchylosed 
to, and fused with, the cephalothorax ” ; but when he comes to 
his fuller description of this group he modifies this statement into 
“ abdomen generally anchylosed to the cephalothorax and wore or 
less fused with it ; ” and when he comes to define the Trombi- 
dieae, which are a portion of the group, he says, “ cephalothorax very 
distinct from the abdomen.” I might multiply instances almost to 
any extent. 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 Phalangidae ? A new 
means would probably 'have to be found of distinguishing them. 
Probably one great reason of the whole thing is that if a separate 
abdomen be admitted, then it must also be admitted that in such 
forms as Tyroglyphus cortiealis and numerous others, if not in all the 
Acarina, the two hind pairs of legs are abdominal ; and this clashes 
with the ordinary book definitions of Arachnoidea as having a legless 
abdomen. Nature, however, will not always be bound by classifica- 
tions, and however unwilling some classifiers may be to admit that a 
portion of the body which bears well-developed legs can possibly be 
the abdomen in any of the Arachnid a, it may be that it will prove to 
be so. 
Having endeavoured, then, to get more or less of an idea of what 
an Acarus is we will return for a moment to the single Linnaean 
genus of that name. Now a Linnaean genus has a great sacredness in 
the eyes of many excellent zoologists and its preservation becomes a 
point of honour ; accordingly a great struggle has been made to retain 
the genus Acarus ; but unfortunately its advocates have not been by 
any means agreed as to what creatures should be favoured with this 
time-honoured name. The late Francis Pascoe was a great advocate 
