ANATOMY AND CLASSIFICATION OF THE ARENICOLID^. 499 



has often two lenticular nucleoli. The protoplasm of the cell 

 stains but slightly. The neuroglial sheath was less developed 

 than in A. Grubii^ and neither the radiating glial processes 

 which extend into the cell protoplasm nor the neuro-fibrillas 

 could be demonstrated so clearly as in that species. 



The giant-cells and giaut-fibres do not appear to be differ- 

 entiated in a post-larval specimen 4*5 mm. long. In specimens 

 50 — 75 mm. three such cells occasionally occurred in segments 

 3 — 6, but they would appear to be less regular and no larger 

 than in the individual 17'5 long referred to in the preceding 

 paragraph. In large specimens (250 mm. or even longer) at 

 least one cell with a very distinct process prolonged into the 

 giant-fibre was met with^ close behind the union of the 

 oesophageal commissures. 



In Arenicola cristata we examined sections of seg- 

 ments 11 and 12 and found a single giant-cell in each^ in 

 a position exactly corresponding to the hinder of two cells 

 of A. marina or A. Grrubii (see fig. 81). The preservation, 

 however, of this and other specimens of the species does not 

 permit us to add any histological characters. 



In A. Claparedii there is, curiously enough, no trace 

 either of giant-cells or giant-fibres. 



InA.ecaudata (150 mm. long) we have found sis giant- 

 cells in segments 2 — 7, and probably therefore they occur 

 segmentally throughout the length of the body. They vary 

 somewhat in size, but attain dimensions as great as those of 

 A. Grubii, measuring '08 mm. in length and "048 in depth. 

 The histology of these cells and their connection with the 

 median giant-fibre are identical with those of A. Grubii. A 

 single process arises from the cell, courses laterally through 

 the fibrous part of the cord, and ends in the median giant- 

 fibre. In two post-larval specimens of this species (94 and 

 7*2 mm. long respectively) the median giant-fibre is already 

 developed, but the giant-cells cannot be made out. 



No further description need be given in this place of the 

 giant cells and fibres of A. Grubii. In adult specimens they 

 persist, and are apparently as large and well developed aa 



VOL. 43, PART 3. NEW SEKIKS. MM 



