LINEUS. 257 



difficult to say exactly where the one ceases and the other 

 starts. The dorsal ganglia are closely united with the 

 ventral (PI. L, fig. 5, and PL II., fig. 4). The two dorsal 

 ganglia are connected by a dorsal commissure (PI. I., fig. 

 4) and the ventral ganglia by a much stouter ventral com- 

 missure. The nervous ring thus formed surrounds 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 ventral ganglia, 

 are continued into the so-called cerebral 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 cords, 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 which almost fill the cell bodies. They 

 occur on the dorsal and ventral aspects of the dorsal 

 ganglia (PL II., fig. 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 brain lobes and 

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

 lion cell layer of the latter. They vary somewhat in 

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

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

 in the 



(3) Large cells. These are also of elongated pyriform 

 shape, 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 



probably motor in function. In some Lineidse a yet 



larger type of cell may be present in the ventral ganglia, 



and sometimes also in the lateral cords. They possess 



R 



