SEA-PLSHE1UES LABORATORY. 257 



points where tlio spinal nerves are given off. The cord 

 gives off a pair of nerves situated in each inter-annular 

 groove, and lying below the circular muscles, and in each 

 chaetigerous annulus either a pair of stout nerves or two 

 bundles (right and left) of two to four nerves, which run 

 outwards towards the parapodia, supplying the circular 

 and longitudinal muscles between which they lie (fig, 55). 



In transverse section the cord is usually oval in shape, 

 being flattened from above downwards. The cells are 

 arranged on the ventral and ventro-lateral faces of the 

 fibrous part of the cord. The entire cord is invested by a 

 thin sheath of neurilemma, and the fibrous part of the 

 cord is partially sub-divided into two by a mediau vertical 

 sheet of neuroglia (fig. 54). 



The ganglion cells are chiefly unipolar, and are small 

 and sub-equal, though here and there are larger cells, 

 generally in the neighbourhood of the giant cells. 



The Giant Cells and Giant Fibres — These cells are 

 much larger nerve cells, placed at segmental intervals along 

 the cord (fig. 52). There are no giant cells in the brain or 

 oesophageal connectives ; the first one occurs just behind 

 the point of union of the connectives near the level of the 

 groove between the third and fourth chsetigerous annuli. 

 This cell belongs to the achsetous segment (composed of 

 the third and fourth annuli) immediately following the 

 peristomium. The next cell is found in the annulus 

 behind the first chsetigerous annulus, and the cells in 

 most of the other segments are found in a corresponding 

 position, but they may be a little anterior or posterior to 

 this level. The cells are, therefore, situated close to the 

 posterior limit of each segment. In many of the segments 

 only a single giant cell is present, but in about one-third 

 or oue-half of the segments two cells are found near 

 together, one in front of the other. They are present in 

 T 



