22 
Transactions of the Society. 
Somehow, these exceptional forms always seem particularly 
interesting ; partly, perhaps, because we speculate how they arose. A 
good instance is found in Zetorchestes, a genus of Oribatidae having 
one species only : all other Oribatidae are slow crawling creatures, with 
all the legs of the same character ; in this one species only the fourth 
pair of legs are greatly developed and form leaping organs ; the 
creature jumps like a flea ; yet there are not any intermediate steps 
known between it and other Oribatidae, which it resembles in the 
rest of its anatomy. In all other Oribatidae the legs are arranged in 
two groups each of two pairs; in this species only the three front 
pairs are crowded together and widely separated from the fourth, 
while the acetabula into which the fourth pair are inserted are 
positively gigantic compared to those of the other legs and of other 
members of the family ; this of course is to give scope for the great 
leaping-muscles. 
The two remaining mouth-organs, neither of which are invariably 
present, are the lingua and the epipharynx ; these project into the 
mouth-cavity and spring, the former from the lower, and the latter 
from the upper anterior edge of the pharynx. The former is too well 
known to need comment here, but I fancy that the epipharynx has 
been much neglected, both by entomologists and arachnologists ; 
when well developed it is a membranous, often haired or fringed, 
penthouse overhanging the entrance to the canal — a sort of portico to 
the pharynx. It must surely have some function ; it may be merely to 
guide the food, but I fancy it also serves to exclude particles which 
are too big for the frequently small lumen of the oesophagus. 
The alimentary canal consists, where perfect, of the pharynx, the 
oesophagus, the ventriculus with its caeca, the liindgut, and the 
Malpighian vessels. The pharynx is the great sucking-organ; 
although, of course, considerable variation in details is found in 
different families, yet the principle, and the leading features of con- 
struction, are singularly constant throughout the Acarina ; indeed, the 
affinity is very strong to the arrangement of the corresponding part 
in most other Arachnida. The arrangement in the Acarina is as 
follows. If you imagine a short and tolerably large chitinous tube 
cut in half longitudinally, so as to form two half-tubes like the gutters 
which are put round the roofs of houses, and imagine that one is laid 
horizontally and the other placed inside it, both having the convex 
side downward, and that the edges are fastened together, you have a 
tolerably good idea of the chitinous part of the apparatus. The lower 
half-tube is usually nearly rigid and fixed ; the upper somewhat 
flexible. A number of perpendicular muscles arise from the roof of 
the rostrum, and, descending, are inserted by tendinous attachments 
along the median line of the dorsal side of the upper half-tube. When 
these muscles contract they of course raise the upper half- tube in the 
centre, and instead of being concave upwards it becomes convex in 
that direction, thus leaving a considerable space between it and the 
