480 F. W. GAMBLE AND J. H. ASH WORTH. 



Ill, Ventral Nerve-cord, 



The position of the cord in the body varies in different 

 species and in different parts of the body of the same species. 

 In Arenicola Grubii and A, ecaudata the cord is coe- 

 lomic throughout. In A, Claparedii itis epithelial, or only 

 covered externally by circular muscles, while its ganglion 

 cells lie practically in the epidermis. In A, marina and A. 

 cristata the cord is epithelial in the tail, but is coelomic in 

 position in the branchial and anterior regions. This varia- 

 tion is exactly parallel to the varying position of the cord 

 in Capitellids, with wliich the Arenicolidfe also agree in the 

 structure of their cord. 



There are no signs of a segmentation or ganglionated 

 arrangement of the elements, except the presence of the 

 " giant-cells " at intervals corresponding to the segments. 

 Around these giant-cells there are rather more ganglion cells 

 of the larger type than elsewhere, but this cannot be said 

 to constitute a ganglionic segmentation. Ganglion cells, in 

 fact, occur along the whole length of the ventral and lateral 

 surfaces of the cord, and this continuous covering is only 

 interrupted by the exit of the spinal nerves. 



When the cord has a coelomic position (as in A, Grubii), 

 it is attached by its lower angles and under surface to the 

 longitudinal muscles which are inserted partly into the 

 neurilemma covering, and partly beneath this into the sheath 

 of the more ventrally placed ganglion cells. The vascular 

 supply is poor, and except for a superficial collection of 

 capillaries there are few traces of blood-vessels in the truly 

 nervous structures of the cord. 



In transverse sections the cord exhibits the usual arrange- 

 ment of a ventral cellular and a dorsal fibrous portion. There 

 is a marked development of neuroglia, as a sheet of tissue in 

 a vertical plane separating the right portion of the cord from 

 the left. This sheet of neuroglia is continuous with a net- 

 work of the same tissue which invests the ganglionic layer. 



The neurilemma forms a stout, deeply staining, laminate. 



