ANATOMY AND CLASSIFICATION OF THE ARENICOLID^. 481 



sheatli for tlie whole cord. Here and there it penetrates 

 deeply into the fibrous region, which it subdivides into 

 areas. In A. Claparedii it is very feebly developed, and 

 practically replaced by muscle-fibres of the circular series. 



The neuroglia is composed of delicate nucleated cells, 

 and of fibrillce which invest the nervous elements. The 

 structure of this tissue is more fully dealt with in connec- 

 tion with the giant-cells (p. 486). In horizontal sections it 

 forms a number of deeply-staining fibrillse which run longi- 

 tudinally between the two lateral masses of ganglion cells, 

 and fills up the spaces between these and the giant-cells. 

 The resemblance between these longitudinal glia-fibrillse 

 and the neuro-fibrillar processes of the true ganglion cells is 

 at first sight, in many preparations, a very close one, but 

 the relation of the fibrillae to the cells (ia the case of neuro- 

 glia forming sheaths, while the neurofibrillse enter the sub- 

 stance of the ganglion cells through their process) enables 

 one to distinguish them. 



Ganglionic Covering. — The majority of ganglion cells 

 in the nerve-cord of Arenicola Grubii are unipolar, and 

 are collected into two lateral masses, which at the point of 

 emergence of a spinal nerve become subdivided into four — 

 two median and two lateral. The largest of the ordinary 

 ganglion cells are placed near the " giant-cells," and in 

 most cases their fibrillar processes divide, one part going 

 outwards as a constituent of a nerve and the other breaking 

 up in the fibrous part of the cord. It is difficult to ascertain 

 the detailed structure of these ganglion cells, since even the 

 larger ones only attain a diameter of '02 mm. ; but a distinc- 

 tion may certainly be drawn between these larger cells and 

 the more laterally placed smaller cells ('008 — "01 mm.), on 

 account of the constant difference of their nuclei. The 

 nuclei of the larger pyriform cells measure "012 mm. in 

 diameter, and are clear vesicles with one large (and some- 

 times one small) nucleolus, but with a very slight development 

 of chromatin network. The cell-body is composed of a more 

 deeply staining central region and a peripheral alveolar zone. 



