The anatomy of scalibe^gma inflatujm. 267 



The Nerve-cord. 



The most striking point in Danielsseu's description of the 

 nerve-cord of Scalibregma is the ganglionation. I am 

 unable to find any trace of the segmentation or gangliona- 

 tion of the nerve-cord either in dissections or in sections 

 taken in various planes. Ganglion cells occur, apparently 

 evenly distributed along the whole length of the cord on its 

 lateral and ventral faces, as in Arenicola (Gamble and 

 Ashworth, 1900, p. 480), In most species of Arenicola, 

 however, an indication of the segmentation of the cord is 

 afforded by the presence of giant-cells placed at regular in- 

 tervals along the cord near the posterior boundary of each 

 segment. In Scalibregma there are no such landmarks, 

 giant-cells and giant-fibres are entirely absent. 



The nerve-cord of Scalibregma is not coelomic in posi- 

 tion in any part of its course. It is situated in the body-wall 

 outside the layer of circular muscles, and in close contact 

 with the epidermis (fig. 16). 



The cord gives off a pair of nerves situated in each inter- 

 annular groove in the basal portion of the epidermis (fig. 12). 

 The nerves which lie in the groove immediately behind each 

 chietigerous annulus are larger than the rest. Besides these 

 there is a pair of moderately large nerves given off opposite 

 the middle of each chaetigerous annulus, which also run be- 

 tween the epidermis and the circular muscles. Each of these 

 nerves (fig. 16) gives off (1) a branch passing into the base 

 of the ventral cirrus, and spreading out beneath the epi- 

 dermis ; (2) a branch to the lateral sense organ; (3) a branch 

 to the dorsal cirrus. The nerve then continues dorsally 

 along the annulus, gradually tapering, and becoming very 

 difficult to trace. The nerve to each cirrus comes into close 

 contact at one point with, and sends fibres to, the corre- 

 sponding setal sac near its mouth. 



In the posterior portion of the animal the nerve-cord lies in 

 very close relation to the epidermis, which is here very thin. 



