216 bulletin: museum of compakative zoology. 



Their innervation is also essentially the same. In the vertebrate crista 

 an auditory nerve fibre passing from the brain is connected with a bipolar 

 nerve cell in the auditory ganglion, from whence its peripheral fibre ex- 

 tends to one of the epithelial sense cells, ending with a slight enlarge- 

 ment in close proximity to, or in contact with its base. The single fibre 

 supplying each end-organ is never directly connected with the cell, nor does 

 it ever run through it to the hair itself. The only difference between the 

 peripheral endings just described, and those of the otocyst, is that in the 

 hairs of the latter the fibres end free in the base of the hollow sliaft, at 

 the point where, from the structure of the hair, the greatest stimulus 

 would be produced ; while in the vertebrate end-organ the nerve process 

 is applied to the convex under-surface of the basal cell, which would 

 transmit stimuli with an equal degree of intensity to fibres in contact 

 with it at any point. 



The otoliths of the vertebrate ear are formed by secretion, while 

 those of the crustacean otocyst are largely granules of sand taken into 

 the sac from the exterior. In some Crustacea, however, such as the 

 Mysidfe, and in many other invertebrates, the otoliths are formed within 

 the sac. 



In all decapods the innervation of the otocyst hairs distinctl}' diff"er8 

 from that of the olfactory bristles, not only as to peripheral termina- 

 tions, but also in the number of nerve elements supplying each hair. 

 As has been previously noted, the stimulus is transmitted by specialized 

 cells or hairs to the nerve fibres of both the otocyst and the vertebrate 

 ear, and is never applied directly to their endings. In either case only one 

 nerve element is usually in contact with the terminal sense cell, and this 

 is apparently ample to carry the isolated nervous message to the brain. 



With the olfactory sense it is different ; in both vertebrates and 

 Crustacea the chemical stimuli which produce the olfactory sensations 

 act directly upon the nerve cells or their terminal fibres. In vertebrates 

 portions of the nerve cells are exposed at the surface of the olfactory 

 epithelium. In Crustacea peripheral fibres from the ganglion cells of 

 the olfactory nerve end free in the hollow, perforate bristles. In Nereis 

 and the earth-worm, Langdon ('95, '00) has shown that the processes of 

 the olfactory cells end free upon the surface of the cuticula, and com- 

 pletely exposed to chemical stimuli ; a similar condition has been shown 

 by Lewis ('98) to exist in two polycha^tous worms of the family 

 Maldanidse. 



The large numbers of nerve elements ending in each olfactory tube 

 or bristle of decapod Crustacea may be accounted for by the fact that 



