16 
Transactions of the Society. 
papers also issued from Colin’s laboratory at Vienna in 1888, viz. 
Winkler’s excellent paper on the anatomy of Gamasus, and Schaub’s 
on that of Hydrodroma (Hydrachnidse). Finally, there are my own 
works in 1883, on the anatomy of the Oribatidse ; in 1892, on that of 
the Gamasidae ; and in 1895, on that of Thyas petrophilus, one of the 
crawling Hydrachnidse. There are also contributions of less import- 
ance by Kramer, Haller, and Donnadieu, besides the careful descrip- 
tions of external anatomy and mouth-organs by Robin and Fumose, 
Megnin and others. 
The leading characteristic which impresses itself upon the mind 
in studying the anatomy of the Acarina is centralisation ; all the 
important functions seem to emanate more from a single centre, and 
all the important organs seem more crowded together, than is usual in 
other groups of living creatures. Division into somites or segments 
is. utterly lost, except in the very young stages of a few species, and 
except so far as any idea of it may be obtained from the number of 
appendages, and from the embryology ; even these often lead to some 
very puzzling results ; thus practically all Acari, except Phytopti, 
where the hind legs are abortive, are born with three pairs of legs, 
and only acquire the fourth after the first change of skin ; but Winkler 
has observed, I think correctly, that the young embryo of Gamasus 
has four pairs, one of which atrophies before birth, to be again 
developed at a later stage of existence, and this has lately been con- 
firmed by Supino in Ixodes. This centralisation is carried so far in 
the external form, that in the typical Hydrachnidse, such as Piona or 
PLydrachna , the head, thorax, and abdomen are one mass ; and no dis- 
tinction between one and the other can be found. The text-books 
almost invariably give this as the character of the whole of the Aca- 
rina, and the point which distinguishes them from other Arachnida ; 
but this is an error : in the great majority of families the distinction 
between cephalothorax and abdomen is perfectly clear, except in the 
view of those who consider that nothing can be an abdomen in the 
Tracheata which is not legless — a view which I am not able to con- 
cur in, as I do not see any reason why abdominal appendages should 
not persist as well as those on the anterior portions of the body ; 
consequently it seems to me that as a definition of the Acarina the 
coalescence of cephalothorax and abdomen entirely breaks down. 
The centralisation is best shown by the so-called brain, or great 
nerve-centre, of an A cams ; instead of the chain of ganglia found in 
Insects and most other Arthropods we have here practically the 
whole united into one ganglionic mass surrounding the oesophagus, 
and from this all the principal nerves arise. As during the past year 
I have perhaps been able to carry the knowledge of the springing of 
the nerves a little further than it had previously attained to I will 
show you a picture of the brain of the creature investigated, namely 
Thyas petrophilus, a new species of Water-mite. 
When I sat down to compose this address I had hoped to give you 
