266 PROFESSOR MARSHALL. 



In the centre of the visceral mass is a plexiform structure (fig. 1, g), 

 the real nature of which has been much disputed, but which, according 

 to Ludwig and P. H. Carpenter, is part of the vascular system from which 

 branches are given to all parts of the body, and among others a radial 

 ventral vessel down each arm in the substance of the subepithelial 

 band. This central plexus passes down through the central canal 

 formed by the First Radials, passes through a hole in the middle of 

 the rosette, and enters the cavity in the centrodorsal plate, where it 

 expands to form a sac divided by vertical septa into five radial com- 

 partments, and hence called the chambered organ (fig. 1,/). 



The chambered organ is surrounded by a thick fibrillar investment 

 (d) known as the central capsule, and this is in connection with a 

 system of fibrillar bands which run down the arms in the substance of 

 the calcareous joints, and are hence called axial cords (figs. 1, 2, 3, a). 

 The connection between the central capsule and the axial cords 

 is rather complicated ; but it is necessary to describe it in some 

 detail, as it is with these parts that we shall be specially concerned 

 later on. 



The central capsule is lodged as we have seen in the hollow of the 

 centrodorsal plate and is covered on its ventral surface by the rosette ; 

 it forms a complete investment to the chambered organ (fig. 1) except- 

 ing where it is perforated by the central plexus in the middle of the 

 ventral surface. The dorsal and lateral walls are, as shown in the 

 figure, thicker than the ventral. 



From the dorsal surface are given off processes to the cirri (fig. 1, e), 

 each of which is traversed down its centre by a vessel derived from 

 the central plexus. 



From the margin of the central capsule arise five short interradial 

 processes (figs. 1 and 3), which, passing ventralwards and slightly 

 outwards, bifurcate into right and left branches between the centro- 

 dorsal plate and the First Radials. These branches, diverging from 

 one another, enter the substance of the First Radials and then unite 

 in pairs, the right branch of one interradial stem uniting with the left 

 branch of the one next to it, to form five stout radial nerves (fig. 3) 

 which run outwards in the substance of the First and Second Radials. 

 On reach' ig the boundary line between the Second and Third Radials 

 each of these radial cords divides into two branches right and left, 

 which, traversing the Third Radial, enter the right and left arms 

 respectively of the pair, along which they pass as the axial cords 



