258 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



the tail, but are not usually recognisable in the first few 

 segments, which are very small, and probably only 

 recently formed. Typical giant cells may be seen in some 

 of the middle and posterior caudal segments, but careful 

 search has failed to reveal them in a considerable 

 number of the tail segments of the two worms examined. 

 In the mid-dorsal region of the cord there are one, two 

 or three giant fibres seen in section (figs. 35, 36). 

 Anteriorly there is only one, in the middle region of the 

 body either two or three, and in the tail usually one. At 

 first sight they appear to be tubes with distinct walls and 

 homogeneous contents, but by suitably staining longi- 

 tudinal sections the contents are found to be distinctly 

 fibrillar, but the fibrils are so fine that they are scarcely 

 recognisable in transverse sections. The wall of the tube 

 is composed of nucleated cells. These giant fibres arise 

 from the giant cells in the cord. This is not so easily 

 demonstrated in Arenicola marina as in A. f/rubii, in which 

 the cells and fibres are larger (fig. 53). In the latter 

 species each giant cell gives off a stout process, which runs 

 in a somewhat sinuous course through the fibrous part of 

 one side of the cord towards the dorsal surface. Shortly 

 after leaving the cell the process gives off one or two 

 branches, which pass into and ramify in the fibrous 

 part of the cord. The process then passes dorsally 

 and enters either the median or, more usually, one of the 

 lateral giant fibres. The single median fibre present in 

 the anterior region of the cord arises from the first giant 

 cell situated just behind the union of the oesophageal con- 

 nectives. In those portions of the cord where two or three 

 giant fibres are present there are connections between 

 them, generally in each chsetigerous annulus. Branches 

 from one or other of the giant fibres may occasionally be 

 traced into the spinal nerves. 



