DR. ARTHUR FARRE ON THE ORGAN OF HEARING IN CRUSTACEA. 237 
A valvular orifice (Plate X. fig. 11.6.) upon the upper surface of the joint contain- 
ing the organ leads to the interior of the sac. This orifice is guarded internally by 
a double group of processes (fig. 15. b.) projecting forwards towards the aperture, as 
in the Lobster. 
This species is so small that the examination and dissection of the parts require 
great delicacy in handling them, as is also the case with the next species. All these 
dissections require to be made under water. 
In the Hermit Crab ( Pagurus streblonyx ) the form of the vestibular sac differs 
again from both the preceding species. It is somewhat cordate, having the base at- 
tached near the external orifice and the apex projecting backwards, as in the Lobster, 
Plate X. figs. 5, 6, 7- 
The processes are not so regularly arranged in this as in the former genus, they 
are also shorter and more pubescent, Plate X. figs. 8, 9, 10. In all other respects the 
organ resembles those already described, having the same valvular aperture (fig. 3. a.) 
leading into the sac, the same arrangement of hairy processes guarding this orifice 
(figs. 7, 8. a.), and the same siliceous particles in the cavity of the sac (fig. 8. 6.). 
In Palinurus there is an obvious degeneration with regard to the auditory appa- 
ratus, agreeing perhaps with the comparatively rude form and less perfect endow- 
ments of this genus. The organ also is far smaller in proportion to the size of the 
animal than in other genera. 
The vestibular sac forms here a small lappet (Plate X. fig. 19. a.) of a tough 
leathery consistence, hanging into the interior of the joint of the antenna, of which 
it occupies but a small portion (fig. 19.). The sac is not transparent, and the processes 
are few in number and arranged with little regularity (figs. 20, 21.). The siliceous 
particles are of larger size (fig. 20.), and the aperture (fig. 18. a.) proportionally 
large and free, and its situation more prominently marked externally than in other 
genera. 
These dissections will suffice to show that there exists in several genera of the class 
Crustacea an organ of hearing of very delicate conformation and remarkable uni- 
formity of type. How far the existence of this organ may be general throughout the 
class I have not had opportunity to determine. It certainly does not belong to all 
the Macrourous genera, as no trace of it is to be found in Squilla ; nor have I found 
it in any of the Brachourous decapods, but of these I have examined only one or two 
species. 
We recognise, however, in this structure all the essential parts of an organ of 
hearing in its primitive form ; a distinct acoustic nerve from the supra-cesophageal 
ganglion, terminating in a plexus which is expanded upon a vestibular sac. In this 
sac nature seems to hint at the formation of a cochlea in the little twisted appendix 
so distinctly visible both in Astacus marinus and in Pagurus, Plate IX. figs. 5 and 6, 
Plate II. figs. 5, 6. These parts, constituting a membranous labyrinth, are surrounded 
and protected by an external case, in which anatomists have already traced the type 
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