PRENTISS : THE OTOCYST OF DECAPOD CRUSTACEA. 203 



a. Number of Nerve Elements to a Single Bristle. The number of 

 cells aud fibres for the whole sac could not be determined with exact- 

 ness, as other sensory elements, supplying tactile hairs, are mingled with 

 those of the otocyst. But in the short transverse row of large hairs, 

 the cells and fibres are sufficiently isolated to allow of their being 

 counted in serial sections. There are but nine hairs in that row, and if 

 the nerve elements supplying them were twice as numerous, it would be 

 at once apparent. The cells always occur singly, and their fibres run 

 separately and parallel with one another to the bases of their respective 

 hairs (Plate 7, Fig. 33). The number of each was counted many 

 times, and it is certain that the number of ganglion cells and peripheral 

 fibres exactly equals the number of hairs. Whole preparations of these 

 nerve elements stained with methylen blue gave regularly nine ganglion 

 cells and fibres supplying the nine sensory hairs. In these few otocyst 

 hairs, at least, there is, then, but a single nerve element supplying each. 

 In the tactile hairs of the scaphognathite of the second maxilla, many 

 methylen-blue impregnations gave conditions like that shown in Figure 

 34 (Plate 7), only one sensory nerve element being stained. In the 

 short spike-shaped bristles found on this same appendage, from three 

 to five ganglion cells (Plate 7, Fig. 32, cl. gn.) were usually found sup- 

 plying each bristle. 



In the olfactory bristles of the antennule, the conditions were the same 

 as those already described and figured for Palsemonetes, though fewer 

 elements compose each spindle-shaped group of cells. 



b. Peripheral Terminations. No branching of peripheral nerve 

 fibres was observed in any sensory elements, though many were 

 traced the whole length of an appendage. In Cambarus the fibres 

 end always at the base in the otocyst hairs (Plate 7, Fig. 33). There 

 is often a marked increase in the diameter of the fibre near its termina- 

 tion, caused either by the staining of its sheath at this point, or by a 

 partial separation of the component fibrillce. Tactile hairs show similar 

 conditions in their nerve endings (Plate 7, Fig. 34). 



The fibre strands of the olfactory bristles were, on the contrary, traced 

 into the shaft some distance, where they apparently end free. Thus in 

 the crayfish, we have a distinct difference in the innervation of the two 

 types of sensory hairs, which serves to confirm the statements made 

 concerning the conditions in Palsemonetes and Crangon. 



c Central Terminations. The otocyst nerve in Cambarus is large 

 enough to be dissected out and traced to the ventral side of the brain, 

 which it enters lateral to the larger anteunular nerve. Its point of en- 



