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Transactions of the Society. 
The so-called brain, or great nervous ganglion, of an Acarus is 
doubtless derived from a supra-oesophageal ganglion united by broad 
commissures to a sub-oesophageal ganglion; but, as in many other 
Aracbnida, the fusion of the parts has been so complete, and the 
commissures have become so broad and short, that the original for- 
mation is lost, and the whole appears to be one ganglionic mass, 
which is penetrated by the oesophagus; still, in the Acarina, the 
traces of the original distinction between the supra- and sub- 
oesophageal ganglia are plainly seen in some families, while they are 
completely lost in others ; this produces a considerable variety in the 
form of the brain, although all are upon the same principle. I 
propose shortly to refer to these differences in this paper, and also to 
endeavour to indicate something of the comparative size of the great 
nervous centre in selected specimens of a few of the great families. 
The brain of the Oribatidae is constructed upon the same principle 
as the brains of other Acarina which have been described ; but the 
first difference which strikes the observer is the very much greater 
proportionate size of the opening through which the oesophagus 
passes as compared with that in the brain of other families ; this will 
be readily seen on comparing fig. 1 of the brain of Leiosoma palma- 
cinctum (Oribatidae) with, for instance, fig. 5, which is that of 
Gamasus terribilis (Gamasidae). The reason for this is obvious ; the 
Oribatidae are vegetable feeders and devour a considerable quantity of 
solid food ; the Gamasidae, and most of the families in which the 
brain has been described, are predatory creatures, living entirely by 
sucking the blood of their victims, and never swallow anything 
except liquid ; it is evident therefore that the Oribatidae would 
require a much larger oesophageal passage than the Gamasidae 
or the Trombidiidae. The only other family yet investigated in which 
the opening in the brain for the passage of the oesophagus is anything 
approaching the size that we find in the Oribatidae is that of the Tyro- 
glyphidae (see fig. 12, Glyciphagus jolatygaster), and these again are 
vegetable feeders and eat solid food, their food being mostly consumed 
in a dry state ; still, the opening even in the Tyroglyphiflae is not so 
large as in the Oribatidae. Of course in this, and all similar remarks 
in this paper, when I speak of the characters of the brain in 
a family it must be understood to mean in such species of that family 
as have hitherto been examined, including those dealt with in this 
paper. 
The next character which will be observed, and it is a very 
strongly marked feature in the two species which I have good sections 
of, is that -while in most of the known families the greatest measure- 
ment of the brain is in an antero-posterior direction (see figs. 5, 6, 
7, &c.), in the Oribatidae this is far the smallest of the three dimen- 
sions, while the depth considerably exceeds both the length and the 
width ; this produces a sort of shield-shaped brain on edge in the 
body. This form does not follow that of the body of the creature ; it 
