PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 185 
2. Innervation of the Otocyst. 
As already noted, the brain, or supra-wsophageal ganglion, is less 
than a quarter of a millimetre distant from the ear sac. The nerve 
supplying the hairs of the otocyst is thus comparatively short, and can 
be traced in a single section from the central to the sensory termination. 
Figures 4 and 12 (Plates 1, 3) show its general course after leaving the 
brain. Its sensory ganglion lies directly beneath the posterior end of the 
sac. The nuclei of the nerve cells of the ganglion are situated about 
0.25 mm. back of the hairs which they innervate, grouped irregularly 
together ; the peripheral fibres of the cells run somewhat parallel to one 
another, then spread out radially to the different hairs of the circle which 
they supply (Plate 3, Fig. 12, for. pi’ph.). 
There are three questionable points to be settled in regard to the 
innervation of the otocyst, and the same is true for the sensory bristles 
of decapod Crustacea in general. 
a. Is each hair supplied by one nerve fibre and sensory cell, or by 
many ? 
b. How do the peripheral fibres terminate? Do they attach them- 
selves to a sense cell, or to some part of the hair, or do they end free? 
If this latter be the condition, does the fibre terminate at the base of the 
hair, or at its very tip ? 
ec. Where do the fibres end in the central nerve organ, and how? 
For the determination of these questions, it is important to compare 
the conditions found in all kinds of sensory bristles. Because different 
types of hairs have been used in various Crustacea for the study of the 
nerve terminations, and this difference in kind of material employed by 
various investigators may account for the very diverse conclusions they 
have drawn. 
All sensory bristles of decapod Grubtepes can be divided into two 
general types : 
(1) Tactile bristles (Plate 2, Fig. 8) have typically a long, straight, 
plumed, attenuate shaft, attached at the base by a thin spherical en- 
largement, which allows great freedom of movement. 
Auditory hairs, so called, are merely modifications of these, for all 
gradations between the two exist. Tactile hairs are found on nearly all 
the appendages, and on some parts of the body. 
(2) Olfactory bristles (Plate 4, Fig. 13, set. olf., and Fig. 14) are short, 
cylindrical, or slightly tapering, and firmly attached as compared with 
tactile hairs, there being no marked basal enlargement. At the tip, the 
