PRENTISS : THE OTOCYST OF DECAPOD CKUSTACEA. 185 



2. Innervation of the Otocyst. 



As already noted, the brain, or supra-oesophageal 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 iiregularly 

 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, fbr. 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 1 Do they attach them- 

 selves to a sense cell, or to some part of the hair, or do they end free 1 

 If this latter be the condition, does the fibre terminate at the base of the 

 hair, or at its very tip 1 



c. 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 Crustacea 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 tapei'ing, and firmly attached as compared with 

 tactile hairs, there being no marked basal enlargement. At the tip, the 



