The President's Address. By A. B. Michael. 23 
lower half-tube; as the edges are anchylosed a partial vacuum is 
formed in this space ; the food, generally liquid, rushes into it ; the 
muscles then relax, the upper half-tube returns by elasticity to its 
normal position, thus once more greatly diminishing the space ; but, 
as the entrance to the mouth is closed by a valve, the food contained 
in the lumen of the pharynx is driven backward through the oeso- 
phagus into the stomach. In the Oribatidae, which swallow some 
solid food, the lower half-tube also is more or less flexible, and can be 
bent down by muscles arising from the floor of the rostrum, thus still 
further enlarging the hollow of the pharynx. In a paper which I 
read before the Zoological Society this year I described what seemed 
to me an interesting variation which I lately discovered in the 
anatomy of the pharynx, in the new species of Water-Mite ( Thy as 
petrophilus) before referred to. In the Acari a considerable propor- 
tion of the muscles are attached to their point of insertion by tendons, 
each muscle being attached by a single tendon ; this is usually the 
case with the muscles which raise the roof of the pharynx ; but in 
this particular species these levator muscles are broad and strap-like ; 
each is attached to the roof of the pharynx by six to eight short 
tendons, slightly radiating ; it will be seen at once what a grasp of 
the surface to be raised this gives to the muscle. 
In some Gamasidae there is a substantial variation in the detail of 
the pharynx ; the lumen of the organ in transverse section is like a 
tri-radiate sponge-spicule instead of the simple crescent ; and not only 
are there distensor muscles inserted into the roof, but also in the sides 
of the pharynx ; while the constrictor muscles pass from the apex of 
one ray to that of the other, i. e. from one ridge to the next in the 
uncut organ. 
The oesophagus is a long but simple tube running longitudinally 
right through the centre of the brain ; it is of small diameter in the 
predatory species which only suck blood, such as Trombidium, but is 
larger in vegetable-feeders such as the Oribatidse, in which family it 
is generally provided with numerous ring-muscles. 
The ventriculus, or stomach, with its cseca, is the great digesting- 
organ, but the relative importance of the central viscus and its caeca 
varies in different families; the cseca are not usually numerous, as 
they are in many Spiders and some other Arachnids, but they are 
often very large. Taking the Oribatidse, which are vegetable-feeders, 
as a type to start with, the ventriculus itself is always a sac of very 
substantial size, usually provided with two large csecal appendages 
directed backwards ; these in such a species as Nothrus theleproctus 
may be longer, and the two together broader, than the central organ ; 
they are apparently partly food -containing, and partly glandular 
structures ; and, in many Acari, exhibit peristaltic movements. In 
the Gamasidse (animal feeders) the ventriculus itself is generally quite 
a small organ, which, in effect, only forms a centre of communication 
between the caeca, which are more numerous than in the Oribatidae. 
