PKENTISS : 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 olfiictory 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 theoiy, 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 

 fibrilUe, — the fibrillar structure of the fibres is confirmed by my prepara- 

 tions, though no fibrillse 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 Hirudineae, 

 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 fibrillee in a motor fibre than extended into 

 its central ganglion cell, and also, that some fibrillae entered the fibre by 

 one branch and at once passed out by another, in no case did he trace 



