TUBIFEX. 339 



nuclei are oval in shape, and the cells to which they 

 belong are considerably flattened. The muscular layer 

 has already been sufficiently described. 



The brain is composed of both nerve cells and nerve 

 fibres, the cells being disposed dorsally and laterally, 

 while the fibres occupy the ventral and central parts of 

 the brain. The nerve cells, in section, appear as almost 

 rounded bodies with clearly marked rounded nuclei. The 

 nerve fibres are embedded in a transparent matrix, and 

 when cut transversely give the brain, in the region in 

 which they are, a finely granular appearance. 



The nerve cord, as already mentioned, can be 

 divided into connectives and ganglia, the ganglia being 

 represented on the surface of the cord as swellings between 

 the connectives. In sections it can be seen that the 

 difference in size of the cord in different parts is due to 

 the presence or absence of nerve cells. The connectives 

 are formed only of nerve fibres, whereas the ganglia 

 possess in addition a large number of nerve cells, their 

 lateral and ventral position being different from that 

 which they occupy in the brain (PL VII, fig. 46, n.c; /.). 

 The mass of nerve fibres, particularly in the ganglia, is 

 divided up into different regions by the interposition 

 between the nerve fibres of a delicate fibrous layer, which 

 appears to be a continuation of the connective tissue 

 sheath which completely surrounds the cord (fig. 46). 



As in most Oligochaeta, giant fibres are to be seen in 

 the nerve cord throughout its length. These occupy the 

 same position in the cord as do those of Lumbricus, that 

 is, they lie close to its dorsal surface. The number of 

 fibres present, however, varies in different parts of the 

 body. Anteriorly, just behind the brain and for several 

 segments, the cord contains only one giant fibre, and this 

 lies in the mid-dorsal line. More posteriorly there are 



