PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 189 
In the olfactory bristles the cells are situated about 0.40 mm. posterior 
to the bases of the hairs, and their peripheral nerve fibres, stained by 
methyien blue, were traced tn almost every preparation, some distance 
into the shafts, though in the tactile hairs of tie same appendage no fibres 
could be followed further than the base. Figure 13 (Plate 4) shows 
the olfactory endings, some of them extending half the length of the 
hair shaft, but none as far as the tip; nor was such a condition ever 
found, although a great number of preparations were examined. The 
direct evidence of preparations shows, then, that the peripheral nerve 
endings are different for the different types of hairs. The fibres terminate 
in the enlarged base of tactile bristles, while in olfactory hairs they end 
Sree in the shaft itself. 
This direct evidence is strengthened by other structural conditions. 
(1) Owing to the rigidity of the hair shaft and its delicate basal 
attachment, a mechanical stimulus applied to a tactile hair would be 
apt to proluce its strongest effect at the base. Therefore we should 
expect to find the nerve termination at this, the point of greatest stimu- 
lation. The innervation of the tactile hairs of vertebrates extends only 
to the base, yet the slightest touch of the hair tip stimulates the nerve 
ending. 
Similarly, in the otocyst hairs the point of greatest stimulation must 
be at the base. The hair tips are so entangled with each other, and with 
the otoliths resting upon them, that a stimulus applied to one must 
affect them all. If this stimulus is caused by the shifting of the weight 
of the otoliths resulting from a change in the direction of the pull of 
gravity, it will affect the delicate, labile articular membrane at the base 
of the hairs far more vigorously than the part of the shaft attached 
to an otolith, or entangled with the tip of another hair which is so 
attached. i 
In the olfactory hair, on the other hand, the chemical stimulus finds 
access through the permeable tip, and, traversing the cavity of the shaft, 
comes at once into contact with the terminations of the nerve, which 
here, as we have seen, runs some distance toward the tip of the hair. 
This, then, is a condition of affairs which, in view of the function of the 
olfactory hairs, we should reasonably expect. 
(2) The conditions during hair formation are very unfavorable to the 
assumption that the nerve fibres extend to the tips of the tactile and 
auditory hairs. In adult Palemonetes, a month at least before ecdysis 
takes place, the matrix cells withdraw their processes to the basal por- 
tion of the hair, leaving the upper part of the shaft empty. As the 
