278 
Transactions of the Society. 
The shape of the brain in the Gamasidae, with its great extension 
in an antero-posterior direction along the oesophagus, is very charac- 
teristic, fig. 5 ; Gamasus terribilis is very like that given by Winkler 
in his pi. iii. fig. 9, for another species of Gamasus, and the extent 
to which the sub- oesophageal ganglion is developed backward as 
compared with the supra- oesophageal is a well-marked feature; 
this is not found in the brain of Sejus togatus, hut this is a very ab- 
errant species — and the main character of the Gamasus brain, viz. its 
extension along the oesophagus, is well maintained. 
Finally, it may be interesting to compare the size of the brain in 
the different families of Acarina. I have given at the end of this 
paper a table showing the relative size of the brain to the whole 
creature in selected species from some of the principal families and 
sub-families, and I have given in the plate drawings of median sagittal 
sections of the brain in all these species. I have purposely, in most 
cases, selected species where the brains have not previously been 
figured, as adding more to the general stock of information than if I 
had copied such few figures as exist by other authors. I was of course 
confined to those species of which I happen to possess good serial 
sections. The linear measurements have all been carefully made by 
me from such sections, and may, I think, he relied on as fairly 
correct. In every instance the measurements of the brain and the 
whole creature have been made from the same individual. 
The calculations of volume can only be considered as approxima- 
tions. I am indebted for them entirely to the kindness of Mr. E. M. 
Nelson, who has carefully prepared them from my measurements 
and drawings ; the last three columns of the table are entirely 
Mr. Nelson’s work, not mine. Of course, to obtain absolute correct- 
ness as to the volume of such irregularly shaped microscopic bodies 
as the brains of Acarina, and the Acarina themselves, it would be 
necessary to follow Mr. E. 1\ Newton’s plan of drawing each one 
of the series of sections greatly enlarged to scale upon sheets of 
material of proportionate thickness, and then cutting out the various 
drawings and joining them so as to obtain a model of substantial size ; 
if this model be made in material heavier than water the model could 
first be weighed in air and then in distilled water at a known tempera- 
ture ; the difference between the two weights would give the weight 
of water displaced ; from which its bulk could be calculated ; or a 
mould could be prepared from the model and filled with sand, which 
could be measured. This is the exact method, but it would be 
immensely laborious where there are a considerable number of figures, 
such as those included in the table ; moreover, although the brain, 
which is a solid organ, would probably give a correct model by this 
process, the whole creature undergoes shrinkage and expansion under 
the action of the reagents necessary to prepare it for section-cutting, 
and its form gets more or less altered, so that the model of the whole 
creature would only be an approximation after all, probably not more 
