242 
Puate VII. 
Fig. 52. Plan of the nerve cord of a young specimen 
about 25 mm. long to show the position of the giant cells. 
Transverse lines mark the posterior limits of the segment, 
the numbers of some of which are given at the side. The 
cells are drawn too large (x 40) in proportion to the width 
of the cord ( x 25), and the latter is about twice too broad 
in proportion to its length (x 12). The first giant cell is 
situated in the achewtous segment which lies behind the 
peristomium. There was no giant cell in the sixteenth 
chetigerous segment of this specimen. Gnant cells are 
not distinguishable in the small anterior tail segments, but 
a few are present in the more posterior segments. One is 
shown in the twelfth caudal segment. 
Fig. 55. Transverse section of the nerve cord of 
Arenicola grubii. The section shows the dorsal fibrous 
part of the cord, in the mid-dorsal region of which are 
three giant fibres, the nuclei in the sheaths of which are 
shown. In the ventral portion of the section note the 
numerous small nerve cells, the nuclei of which are shown 
on the right and left; a bundle of pyriform ganglion cells 
whose processes are directed into the fibrous part of the 
cord; and a giant nerve cell. The protoplasm of the latter 
is reticulate and its nucleus vesicular. The cell gives off 
a large process which, after sending branches into the 
fibrous part of the cord, enters the right lateral giant fibre 
at the point where there is a transverse connection between 
the two lateral giant fibres. The neurofibrille of the 
process are shown. This drawing of the giant cell and its 
process was obtained by superposing camera drawings of 
the four consecutive sections in which these structures 
occur. x 210. 
Fie. 54. Transverse section of nerve cord of Areni- 
c 
