The President's Address. By A. D. Michael. 25 
parative physiologists have been agreed that they are excretory 
(urinary) organs. Now there is not any doubt whatever that the 
malpighian vessels of Acari and some other Arachnida are excretory ; 
they are filled with the unmistakable opaque, white, crystalline matter, 
and with that only ; hence the analogy. But Prof. Lowne, in the 
recent edition of his work on the Blow-fly, is inclined to return to 
the hepatic function in Insects ; if he he correct, what becomes of the 
analogy ? 
If we turn to the great Acarine group before spoken of, of which 
Trombidium may be considered the type, a very interesting question 
arises as to the hind-gut and Malpighian vessels. In 1878, Croneberg, 
in his great work in Russian upon Eylais extendens, one of the 
Hydrachnidae, and also in 1879, in his memoir on Trombidium, 
pointed out that in this group the hind-gut and Malpighian vessels 
were not separate organs, but were represented by a single large tube 
running straight down the middle of the body in the ordinary position 
of the hind-gut, and ending in what in other Mites would be called 
the anus ; but 'that this tube always contains the white excretory 
matter only, and never any trace of food-balls. Croneberg positively 
asserted that in both creatures the ventriculus ends blindly and has 
not any connection with the excretory tube. Henkin, in his work on 
Trombidium, agreed with Croneberg as to the contents and position of 
the organ, and confessed that he had tried in vain to find any connec- 
tion with the ventriculus ; still he thought that it must exist some- 
where or at some time, and that the excretory sac must be regarded 
as a hind-gut. In my study of Thyas I found myself much in 
Henkin’s position ; there was no doubt that the organ which one 
would be inclined to call the hind- gut was filled solely with the white 
excretory matter, without a' trace of the balls of half-digested food 
which are so conspicuous in the hind-guts of other Acari, and I 
utterly failed to find any connection with the ventriculus ; moreover, 
the excretory organ, for such it certainly is, whether it be a modified 
hind-gut or not, considerably overlaps the ventriculus, and a good 
length of the anterior end of the former lies on the dorsal surface of 
the latter. On the whole, one would be inclined to say that this so- 
called Malpighian organ was neither more nor less than the homologue 
of the hind-gut, and that in this group of creatures it had become 
modified so as to be chiefly or wholly an excretory organ, either with- 
out any communication with the mid-gut, or with a connection so fine 
or so temporary that it has not yet been detected, notwithstanding 
the careful searches made by several anatomists. Against this view 
there is one strong argument : in 1888, Schaub published a paper 
which emanated from the laboratory of Prof. Claus at Vienna; this 
paper dealt with the anatomy of Hydrodroma, one of the Hydrachnidae. 
In it Schaub agrees with Croneberg that the organ which ends in 
what is called the anus has not any communication with the ventriculus ; 
but he says that this so-called anus is not the anus at all ; that there is 
