The Brain in the Oribatidse, dec. By A. D. Michael. 279 
exact than what Mr. Nelson has prepared from the measurements and 
drawings. I think it probable that if an actually correct model of 
the whole creature, with all its indentations, could be obtained, the 
brain would bear a slightly larger proportion to the bulk of the 
-animal than that given in the table. 
The species selected are not any of them very active. I wished 
to avoid the difference which might possibly arise from the larger 
nerve-supply required by a very active creature, and to compare 
members of the different families as far as possible unaffected by this 
cause of divergence. Thus, while some Gamasidae, e. g. Hsemo- 
gamasus hirsutus, are extremely active, Gamasus terribilis is not, 
although the two species are found together under precisely similar 
circumstances ; and though many Trombidiidae, e.g. Erythrseus , and 
some Hydrachnidae, are extremely active, yet Trombidium and Thyas 
are not; again some Trombidiidae are very active, but Glyciphagus 
jplatyyaster is not. 
It will be seen from the table, which doubtless is a sufficiently 
close approximation for practical purposes, that the ratio of the brain 
to the body is lowest in the Tyroglyphidae, *19 per ceut. ; this is 
interesting because they are atracheate creatures, and have always 
been considered the lowest organisms of any family in the table. On 
the other hand, the Gamasidae show far the largest proportion of 
brain of any of the great families ; the ratio of volume of brain to 
body in G. terribilis being 1 • 61 per cent. Megnin, many years ago, 
when the size of the brain was quite unknown, asserted that the 
Gamasidae were the most highly organised of any of the Acarina ; 
certainly the size of the brain in such a species as G. terribilis is very 
remarkable: it will be seen from the table that the brain is over 
a fifth of the length, over a fourth of the width, and over a half of 
the depth of the entire creature ; it is however of irregular form, 
which somewhat diminishes its volume. 
The small size of the brain in Trombidium and the Hydrachnidae 
is not so easily accounted for ; here it will be seen it only amounts to 
about a tenth of the length, a seventh to a fifth of the breadth, and 
a fifth to a third of the depth, while the volume is not much more 
than in the Tyroglyphidae ; yet these are rather highly organised 
creatures, with well-developed eyes, which are absent in the other 
families quoted. The much larger proportionate size of the brain in 
Tetranychus, which has generally been supposed to be closely allied 
to the Trombidiidae, is also difficult to explain. The brain in the 
Oribatidae is small when compared with the Gamasidae, but is con- 
siderably larger in proportion than that of the Trombidiidae and 
Hydrachnidae. 
It may be worth mentioning the usual ratio of the brain to the 
body in the human race. In a series of 278 cases weighed by Sims, 
Glendinning, Tiedemann, and Reid, the maximum weight of the 
brain in the adult male was 65 oz. and the minimum 31 oz. In 
