PRENTISS : THE OTOCYST OF DECAPOD CRUSTACEA. 189 



In the olfactory bristles the cells are situated about 0.45 mm. posterior 

 to the bases of the hairs, and their peripheral uerve fibres, staiued by 

 methylen blue, were traced in almost every preparation, some distance 

 into the shafts, though iu the tactile hairs of tiie same appeudage uo fibrea 

 could be followed further thau 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 

 free in the shaft itself. 



This direct evidence is strengthened by other structural conditions. 



(1) Owing to the rigidity of the hair shift and its delicate basal 

 attachment, a mechanical stimulus applied to a tactile hair would be 

 apt to produce its strongest effect at the base. Therefore we should 

 expect to find the uerve termination at this, the point of gi-eatest 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 

 afiPect them all. If tliis 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. 



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 Pala^monetes, 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 



