difficult to sa}- exactly wlieie the one ceases and the other 

 starts. The dorsal gang-lia arc closely united "vvith the 

 ventral (PI. I., tig. 5, and IM. 11., tig. 4). The two dorsal 

 g'anglia are connected by a doisal commissure (IM. L, fig. 

 4) and the ventral ganglia l)y a much stouter ventral com- 

 missure. The nervous ring thus formed siirrounds the 

 proboscis sheath, and not, as in most worms, the 

 alimentary canal. The posterior ends of the dorsal 

 ganglia, now no longer in contact with the vcMitral ganglia, 

 are continued into the so-called ceiebral organ, which will 

 be referred to under the sense organs later. 



Histologically the general structure of the brain is 

 similar to that of the lateral coids, with the difference 

 that the ganglion cells are not all alike. In the brain 

 three varieties of ganglion cells may be distinguished: — 



(1) Small cells of shortened pyriform shape, the deeply 

 staining nuclei of whicli almost till the cell bodies. They 

 occur on the dorsal and ventral aspects of the dorsal 

 ganglia (PL II., tig. 4), and also in the cerebral organs, 

 and are probably sensory in function. 



{2) Medium sized cells, more or less elongated and 

 pear-shaped. These occur in the ventral l)rain lol)es and 

 in the lateial cords, forming the greater part of the gang- 

 lion cell layer of the latter. They vary somewhat in 

 size, but may be distinguished fiom the next type by the 

 shape of their nucleus, whicli is oval and not spherical, as 

 in tht> 



(•">) Large cells. These are also of elongated jjyriform 

 shap(\ and are found in the dorsal and ventral ganglia, 

 as well as in the lateral cords. 



The larger ganglion cells of the last two types are 

 })robably motor in function. In some Lineidte a yet 

 larger ty])e of cell may be j)r(\scnt in the vential ganglia, 

 and sometimes also in the lateral cords. They possess 



