510 A. MILNES MARSHALL. 



the subepithelial bands of the five radial grooves unite to form a 

 pentagonal ring. 



The tentacles, as described above, are hollow ; their cavities 

 communicate with a longitudinal canal (fig. 2, I) which runs 

 along the arm just below the subepithelial band. These 

 radial ambulacral canals are continued into the disc and 

 open into a circular canal round the mouth (fig. 1) from which 

 short branching canals are given off ending in open mouths 

 communicating with the body cavity. 



Besides the radial ambulacral canal, each arm contains also 

 three diverticula of the body cavity or cselom. Of these the 

 most ventrally situated (fig. 2, m) is called the subtentacular 

 canal and is commonly divided as in the figure by a median 

 vertical partition ; the most dorsally placed canal (fig. 2, n) is 

 called the cseliac and communicates at the end of the arm 

 with the subtentacular. The third or genital canal is 

 placed between the other two and lodges the cord-like genital 

 gland ; it is very small in the arm, but much larger in the 

 pinnules. 



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 E-adials, 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 

 compartments, and hence called the chambered organ 



(fig- 1./). 



The chambered organ is surrounded by a thick fibrillar 

 investment ((i), 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 



