PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 217 
the stimulating chemical substances occur as slight traces only. In order 
that a sensation may be perceptible, apparently a large number of olfac- 
tory elements must be stimulated at once, for the larger their number, 
the stronger should be the sensation produced. ‘The olfactory bristles 
are located on the flagella of the antennules, a position most favorable for 
the reception of chemical stimuli, as the flagellum projects some distance 
in front of the animal and can be kept in constant motion. The number 
of the bristles is limited on account of the small surface to which they 
are necessarily confined, so that, if thousands of olfactory fibres are to 
function simultaneously, large numbers of them must be exposed to the 
chemical stimulus in the same hair. It is possible, too, that different 
nerve elements may be affected by different substances in solution; and 
that consequently many olfactory elements are necessary for each hair, 
in order that different chemical stimuli may be perceived. 
2. The Neuron Theory. 
The conditions found in the sensory nerve elements of the otocyst are 
favorable to the neuron theory, in so far as they confirm the generally 
accepted idea that the nerve fibres are each differentiated from a single 
nerve cell, and that fibre and cell taken together form a trophic unit. 
This conclusion is borne out not only by the structural conditions 
already described, where each fibre is connected with only one peripheral 
ganglion cell, but also by an experiment which I made by severing the 
otocyst nerve proximal to its ganglion ; in this case after the lapse of a 
few weeks degeneration of the sensory fibres took place back into the 
brain. 
As to the modifications of the neuron theory recently proposed by 
Apathy (’97) and Bethe (’98),—that the neurons are connected by 
fibrille, — the fibrillar structure of the fibres is confirmed by my prepara- 
tions, though no fibrillee could be demonstrated in the nerve cells. In 
regard to the definite connection of the neurons with each other by con- 
tinuous fibrils, such as Apathy figures and describes in the Hirudinee, 
my preparations gave no positive evidence; but the fact that the cen- 
tral fibrillations of the nerve elements of the otocyst could not be traced 
to determinate endings, makes it quite possible that such a direct com- 
munication between motor and sensory neurons may exist. While Bethe 
proved that there were more fibrillze ina motor fibre than extended into 
its central ganglion cell, and also, that some fibrillze entered the fibre by 
one branch and at once passed out by another, in no case did he trace 
