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Transactions of the Society. 
was the subject of the paper to he laid before them, and in which 
(as it had been very little investigated before) I was able to make a 
few discoveries, I hope of rather more than passing interest. 
The first point which I will mention is the construction of the 
pharynx, which is the great sucking organ in such Acari as live by 
suction ; and the principal interest in what I am about to mention lies, 
I think, in showing how small a variation will sometimes profoundly 
affect the action of the parts. In order that you may understand this 
variation in BdeJla, I will shortly remind you of what the pharynx 
usually is in allied families, such as the Hydrachnidae. Imagine two 
chitinous half-tubes, like the gutter-pipes round roofs, fitting one 
inside the other, and soldered at the edges, the lower one fixed, the 
upper slightly flexible, so that it can be raised by perpendicular 
muscles. When the roof of the pharynx is so raised, a partial vacuum 
is left between it and the floor ; the food rushes in from the mouth ; 
a valve closes the entrance of the pharynx ; then some transverse 
muscles, which run from one upper edge of the half-tubes to the 
other, contract, driving the upper half-tube down on the lower, and 
consequently (as the opening to the mouth is closed) driving the food 
on into the ventriculus. In Bdella the variation is that the roof of 
the pharynx is not chitinised ; consequently the muscles would not 
raise the whole roof, hut each muscle would raise only the piece it was 
attached to. Therefore a new arrangement is made : the muscles, in- 
stead of acting together, act successively, and the food is carried back 
by the undulation produced, much as in a cat’s lapping. Then the 
transverse muscles, if attached to the upper edges as in other cases, 
would not force the upper half-tube down, they would simply crumple 
it ; therefore they have the ends attached below the pharynx, and pass 
in an arch over it, so that when they straighten, also successively, they 
compress the whole pharynx, and restore it to its original condition. 
The pharynx passes into the oesophagus, which in all known Acarina, 
except Bdella, is a mere straight tube. In Bdella alone, I have dis- 
covered that at the oesophageal end of the pharynx there is a ring- 
constrictor muscle, and immediately behind this ring the oesophagus 
bifurcates ; one branch is the ordinary tube to the ventriculus, the 
other leads into a large blind sac at the end of the tubular stalk, 
and this sac is a reservoir in which food is stored before passing into 
the stomach. The interest of this, to my mind, lies not in the 
arrangement itself, but in the fact that it is unknown amongst other 
Acari, and even, as far as I remember, amongst other Arachnida, 
while it is well known in a totally different class of the Arthropoda, 
namely the Insecta, where it is common amongst Diptera, Lepido- 
ptera, Hymenoptera, Ac. How are we to account for the sudden 
reappearance of this organ in one solitary genus so widely removed 
from those groups where it is typical ? Are we to suppose that a 
common want has produced a similar development in both cases ? 
But if so, how is it that none of the other sucking Acari have it, and 
