PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 213 



b. Peripheral Terminations. As seen in Figure 53 (Plate 10), the 

 terminal fibres going to the thread hairs enter the pore at the base of the 

 cup-shaped depression, pass up into the enlargement of the hair shaft, 

 and there end free. In fact, there is in these hairs no functional neces- 

 sity for the further continuance of the fil)re into the shaft. Since the 

 hairs project free into the liquid of the sac, if the otocyst is jarred or 

 tilted, the shaft does not itself bend, but sways backward and forward 

 upon its base. It is therefore at tlie base that the stimulus must mani- 

 fest itself, and it was there in every case that the fibres were found to 

 end. 



In the olfactory hairs, on the other hand, the nerve fibres continue up 

 into the large hollow shafts for some distance (Plate 10, Fig. 52,se<. olf.). 

 The olfactory hairs of Cai'cinus thus difi^er in their innervation from those 

 of the otocyst, both in the number of nerve elements supplying each hair, 

 and in the peripheral nerve endings. In the bristles of the otocyst there 

 is but a single nerve element, and it ends free at the base of the hair 

 without branching. In the olfactory hairs there may be a hundred ele- 

 ments or more which end in the shaft of a single hair. 



c. Central Terminations. Entering the brain in front of, and just 

 median to, the globulus, and ventral to the optic centres, the fibres of 

 the otocyst nerve run straight back and enter the fibrillar mass (Plate 

 10, Fig. 55, rCpil. at.i), called " the neuropil of the first antenna " by 

 Bethe ('97), who has described the central endings of the antennular 

 nerve of Carcinus. The fibres of the antennular nerve end in a connected 

 neuropil just median to those of the otocyst. Bethe judged from his 

 physiological experiments tliat there should be certain fibres from the 

 otocyst ending in the globulus. He was not able to demonstrate such 

 endings with methylen blue, nor was there any evidence of their exist- 

 ence in my preparations. According to Bethe the fibres from the oto- 

 cyst end by the separation of their fibrillse in the neuropil. Lack of fresh 

 Carcinas material prevented the verification of his work, but I have 

 described similar conditions in the shrimp and crayfish. 



d. Histology of the Nerve Elements. As the finer structure of the 

 elements of the central nervous system has been fully described by Bethe 

 ('98), it is unnecessary for me to say anything on that matter, and 

 only a few words need be added here as to the histology of the peripheral 

 nerves and cells. The peripheral nerve fibres are much smaller than 

 in Palsemonetes or Crangon, and are without a myelin sheath. The 

 peripheral ganglion cells are relatively large, averaging 12 jit in diameter. 

 They are of the typical bipolar form, and are much elongated (Plate 



VOL. XXXVI. — NO. 7 4 



