242 DR. A RT HUR FARRE ON THE ORGAN OF HEARING IN CRUSTACEA. 
Fig-. 5. Portion of joint removed to show the vestibular sac. 
Fig. 6. A further portion removed, showing the same more distinctly. The Pores at 
the base of the processes are shown in this figure. 
Fig. 7. The sac removed, showing the ciliated processes on the parietes c and near 
the orifice a. 
Fig. 8. The same laid open. The letters refer to the same parts, b. Mass of sand. 
Figs. 9 and 10. Groups of ciliated processes. 
Figs. 11 to 17- Astacus fluviatilis. 
Fig. 11. Right antenna, upper surface, b. Valve. 
Fig. 12. The same laid open and viewed from below, a. Vestibular sac. 
Figs. 13 and 14. The same. The crescentic row of orifices is seen in both figures. 
Fig. 15. Vestibular sac laid open. Processes and sand at a. Processes at orifice b , 
Fig. 16. Separate process and grains of sand. 
Fig. 17. Olfactory (?) organ. A bristle passed into the orifice. 
Figs. 18 to 21. Palinurus quadricornis. 
Fig. 18. Antenna, showing external orifice, into which a bristle is passed, «, 
Fig. 19 a. Vestibular sac. 
Fig. 20. Detached ciliated processes and grains of sand. 
Fig. 2 1 . Processes in walls of sac. 
