Nervous System of the Chcetopoda. 315 



In Polynoe two median nerve-fibres and one ventral on 

 each side traverse the ventral cord from before backward. 

 The first two distinctly nnite with ganglion-cells in the 

 beginning of the ventral cord ; in the case of the ventral fibre, 

 which appears even in the oesophageal commissures, I have 

 not succeeded in ascertaining the ganglion-cell belonging 

 to it. 



In agreement with Sthenelais there is, moreover, in Polynoe 

 in each segment on each side an enormously large ganglion- 

 cell, which sends its colossal nervous process transversely 

 through the ventral cord into the last of the three nerves 

 starting in each segment, in common with which it runs to 

 the peripliery. 



In Aphrodite and Hermione colossal nerve-fibres of such 

 construction are entirely wanting. 



In conclusion, I will say a few v/ords as to the structure of 

 the ganglion-cells and their relation to the central fibrillar mass 

 of the nervous system. 



The ganglion- cells of the Aphroditeas are without exception 

 unipolar. In the rest of their structure, however, they show 

 an extraordinary variety. Two opposite types especially 

 occur among them. The ganglion-cells of one kind are very 

 faintly granulated and therefore of clear appearance, and gene- 

 rally rather small. Their nucleus always contains several 

 corpuscles of different sizes, and, when stained, is exceed- 

 ingly prominent in the transparent ganglion-cell. They have 

 a pyriform shape and lie, in large packets, close together. 

 The representatives of the second type are very large spherical 

 structures, which immediately catch the eye by tlieir very dark 

 granulation. They possess a large finely-granulated nucleus, 

 and this a single large corpuscle. They are always single, 

 never united into groups. To this type also belong the 

 colossal ganglion-cells. Both kinds of ganglion-cells are 

 destitute of a cell-membrane and lie imbedded in a network 

 of fibres, which everywhere accompany the nervous system 

 and, I believe, originate from subcuticular cells. But while 

 this envelope of subcuticular fibres is very slight in the 

 ganglion-cells of the first type, appearing rather as a thin 

 partition between the closely appressed cells, in the second 

 type, and especially in the colossal ganglion-cells, it is highly 

 developed. 



The processes of the transparent ganglion-cells of the first 

 type run in bundles and interlaced with each other into the 

 nervous system, accompanied by subcuticular fibres, which, 

 however, disappear soon after their entrance. These cell- 

 processes, which are generally very delicate, gradually become 



