The Brain in the Oribatidse , &e. By A. D. Michael. 277 
is true that Leiosoma palmacinctum (figs. 1, 2) is a round-bodied 
creature, but the form is practically the same in Cepheus latus which 
is not by any means round-bodied. Some of the Hydrachnidse are 
very round and short-bodied, much more so than any of the Oribatidse, 
and yet the antero-posterior measurement of the brain, although short, 
is considerably larger in proportion than that of the Oribatidse. I am 
not able to give the details of one of these forms of Hydrachnidse in 
my table as I do not possess good serial sections of one ; but Schaubs* 
in his description of Hydrodroma dispar gives the long axis as 
*174 mm., the width as *116 mm., and the thickness as ’1 mm.; 
his long axis practically corresponds with the depth, and his thickness 
with the length ; he states his whole creature to be about 2 mm. long 
by about 1 * 5 wide, so that the brain is very small, but Schaub does 
not give the depth of the whole creature. 
A third point to be noticed is that the cortical layer of small, 
rounded, deeply-staining cells, which usually constitutes the exterior 
of the brain in Acarina, is thick in the Oribatidse, whereas in the 
Hydrachnidse it is scarcely traceable, and that the fibrous layer 
exhibits a much more homogeneous and less fibrillar character than in 
the Hydrachnidse, Gamasidse, and many other families ; this may 
probably be correlated with the slighter development of the nerves in 
the Oribatidse. Again, it may be noticed that the oesophagus of the 
Oribatidse, in its passage through the brain, is not accompanied by 
trachese, as it is in Trombidium , &c. This is doubtless due to the 
small number of the unbranched trachese of the former as compared 
with the abundant supply of fine trachese found in the latter. 
I will now shortly glance at the relative form and comparative 
size of the brain in a few of the different families; and the first 
observation that occurs is that the form of the brain seems to follow 
the family far more than it follows the shape of the individual species, 
iust as before remarked in the Oribatidse. Thus the Trombidiidae and 
the Hydrachnidse are closely allied families, and on reference to 
figs. 7, 8, 10, it will be seen that the brains are all more or less of an 
approach to a globular form, notwithstanding that the creatures are 
very different in shape, Thy as, fig. 10, being flattened dorso-ventrally 
as compared with such a Trombidium as fuliginosum , fig. 8. 
Cheyletus, fig. 11, is also generally considered to be an allied form, 
but there are very considerable anatomical differences, and here the 
brain shows a tendency to lose its sub-globular form and approach 
a little to that found in Gamasus. Tetranychus , fig. 9, also is 
classified as an allied form, but it is a vegetable feeder, and there 
certainly is an approach to the form of the brain in other vegetable 
feeders, as will be seen by comparing it with the Tyroglyphidse 
(fig. 12), which lead up to the extreme form found in the Oribatidse. 
* “ Uber die Anatomie von Hydrodroma (G. L. Koch), ein Beitrag zur Kenntniss 
der Hydrachniden,” Sitzungsb. d. k. Akad. d. Wiss. Wien, March 1888, Bd. xcvii. 
Abth. i. pp. 98-151. 
