ANATOIMY OF ARENICOLA ASSIMILIS. 751 



nuclei; are found rather nearer tlie middle line in many of 

 the sections examined. The process of each cell is directed 

 dorsally into the lateral portion of the fibrous mass. The 

 spinal nerves arise in the same manner and position as in 

 other Arenicolidae (Gamble and Ashworth, 1900^ pp. 482, 

 483). Giant-fibres to the number of two or three are seen 

 in sections of the branchial region and tail. The giant-cells 

 are regularly arranged, being situated close to the posterior 

 border of each segment. In eight of the nine segments 

 examined there is only one giant-cell per segment, but in the 

 other segment two cells are present near together. The 

 giant-cells are placed in the extreme lateral regions of the 

 cord, and in the first piece of cord which was sectioned they 

 are situated alternately on the right and left sides, i. e. in 

 the seventeenth to twentieth chastigerous segments, and in 

 the first tail segment of this specimen they are situated 

 respectively K, L, E, L, li (see fig. 12). In sections of the 

 same specin^en taken further forwards (tenth to thirteenth 

 segments) this curiously regular arrangement was not found, 

 the cells present in these segments being situated respectively 

 L, L, R R (two cells are present in this somite), R. The 

 average size of the cells is 0'065 mm. long and 0*05 mm. broad 

 and deep. Each cell is pyriform, surrounded by a fibrillated 

 sheath, and sends out usually only one process, which passes 

 at once into the fibrous portion of the cord toAvards the 

 lateral giant-fibre. The protoplasm of one cell, however, is 

 drawn out dorsally into five processes, one of which is much 

 thicker than the others, and may be easily traced into the 

 mid-dorsal region of the fibrous part of the cord. The 

 slender processes are traceable only a very short distance, 

 being lost either between the small ganglion-cells or imme- 

 diately on entering the fibrous part of the cord. There is in 

 most of the giant-cells a more deeply staining area in the 

 protoplasm close to the nucleus, due to the presence of 

 chromophilous granules. This probably corresponds to the 

 similar but better marked ceutrosphere seen in A. grubii 

 (Gamble and Ashworth, 1900, pp. 487, 488, and fig. 7G). 



