236 DR. ARTHUR FARRE ON THE ORGAN OF HEARING IN CRUSTACEA. 
of the under surface of the sac, and spreading round towards the upper surface, where 
it becomes thinner and is gradually lost. On the under surface, however, the several 
fibi •es forming the plexus are easily seen even by the naked eye ; but they require 
great delicacy of manipulation in their dissection, as the plexus is imbedded in the 
soft fleshy substance of the joint, and cannot be fairly exposed until this has been 
picked away grain by grain. On this account, and also from the readiness with 
which the tegument is detached from the shell in Crustacea, it would be impossible 
to trace any nerve-filaments into the hair-like processes, even should they extend so 
far. They may, however, be traced as forming a considerable plexus up to their very 
base, and being most abundant where the processes are principally situated, while the 
centre of the processes is found to be occupied by granules having every appearance 
of nerve-granules, and which are probably derived from the plexus lying immediately 
beneath them. The antennal nerve (Plate IX. fig. 10. b.), which passes through this 
joint along the inner and under side of the sac to supply the rest of the antenna, 
sends off two or three minute filaments to this plexus, probably for the purpose of 
supplying the contiguous fleshy substance. 
Before I proceed to offer a few brief suggestions as to the use of their several parts, 
it may be well to state how far I have found this structure constant in the class 
Crustacea. 
In the river Cray-fish ( Astacus Jluviatilis ) all the parts that are found in the Lob- 
ster exist, with the exception of the fenestra ovalis and its membrane, which appear 
to be peculiar to the latter, as I have not found them in any other species. The 
parietes of the joint are, however, rather thinner and more flexible in their situation, 
although a distinct membrane is wanting. 
The vestibular sac in this species is nearly hemispherical in form, having the convex 
surface directed upwards and the plane surface downwards, Plate X. figs. 12, 13, 
14. a. Upon the plane surface are situated several rows of hairs, as in the former 
species ; the same disposition to a crescentic arrangement is observed, but the row of 
processes in this species forms rather more than the half of a circle (figs. 13, 14, 15. a.). 
The mode in which they are disposed is readily seen upon the under, or flat surface, 
before the sac is opened, where their seat is indicated by a row of minute pores of 
great regularity, the processes with which they communicate being seen through the 
transparent parietes springing from the margin of the pores and projecting into the 
cavity of the sac. 
The sac is here also filled with minute particles of siliceous sand, which in the 
specimen that I examined were so numerous as to fill about one-third of the 
chamber. 
The processes are more delicate than in any other species ; they are sharply pointed 
at their extremity, and swell out broad towards the base, where they contract again ; 
they are also somewhat flattened and have a single row of hairs on either side, extend- 
ing only about half-way down their length (fig. 16.). 
