490 F. W. GAMBLE AND J. H. ASHWORTH. 



we are dealing. In the first few segments and in the last 

 two the process enters into the median giant-fibre. The first 

 cell, placed at the point of union of the oesophageal commis- 

 sui'es^ may be said to be the starting-point of this fibre ; and 

 though, as we shall see, the median fibre gives off branches 

 and fuses from time to time with the lateral ones, yet it may 

 be said to be continuous from one end of the body to the 

 other (see PL 28, figs. 67—73). 



The greater number of the giant-cells, from the fifth 

 segment to the last but two or three, of Arenicola Grubii 

 enter into direct connection, not with the median giant-fibre, 

 but with the lateral ones. In several cases we have noticed 

 that their cell-processes at first behave as do those which 

 join the median fibre; that is to say, they curve upwards 

 towards the dorsal surface, giving off one or two small 

 branches to the cord on their way. Then they enter one of 

 the lateral giant-fibres, usually at a point where it is connected 

 with its fellow by a junction which runs beneath the median 

 fibre (PI. 28, figs. 67—70). The exact behaviour of the cell 

 process at this point varies in different cases, and requires 

 additional methods to those we have employed for its com- 

 plete investigation. It appears almost certain, however, that 

 the cell-process divides in a T-like fashion. The stalk of the 

 T represents the process, and the cross-piece the lateral 

 neurochord, which, starting from the cell process, runs for- 

 Avards and backwards, and at once, or after a very short 

 course, unites with or sends a branch to, the giant-fibre of the 

 opposite side. A single branch or a pair of lateral branches 

 are also given off from the cell process to the fibrous part 

 of the cord just at its point of division into the neurochord. 

 In other cases the cell-process divides ; one small branch 

 entering a lateral fibre, and the rest gradually lose them- 

 selves in the cord without any definite termination. 



In attempting to trace the lateral giant-fibres from their 

 points of contact with the giant-cell processes, we have found 

 that in front of these points up to the chsetigerous annulus, 

 the two fibres anastomose in a perplexing way, and give off 



