536 A. MILNES MARSHALL. 



bands which connect the axial cords together, for if the axial 

 cords are really nerves these connecting bands, which are 

 identical with them in histological structure, must be nerve 

 also, and experiment ought to throw light on their purpose 

 These commissures are of two kinds (cf. fig. 3) : there is, 

 firstly, the great pentagonal commissure in the First Radials 

 which connects together the roots of the radial cords ; and, 

 secondly, we have in each Third Radial a rather complicated 

 connection, by means of a transverse commissure and a chiasma, 

 between the two axial cords into which each radial cord 

 divides. I propose to deal with these two sets of fibres sepa- 

 rately, taking the great pentagonal commissure first. 



Experiment 20. — A specimen was eviscerated, and a 

 needle passed down from the oral surface into the ch ambered 

 organ, and worked about so as to destroy as completely as pos- 

 sible the central capsule and chambered organ (cf. fig. 1). The 

 animal was then returned to the water, and left at rest for half 

 an hour. One of the arms was then suddenly nipped with 

 forceps, when all the arms exhibited active movem ent, though 

 the animal did not attempt to swim. 



This experiment shows that the central capsule does not 

 form the sole physiological connection between the axial cords 

 (nerves) of the several arms. Figs. 1 and 3 show that the pen- 

 tagonal commissure, which is lodged in the First Radials, would 

 not be touched by the operation, and, as it furnishes an ana- 

 tomical connection between the axial cords, it was naturally 

 suspected to be the physiological connection as well. To test 

 this the following experiment was made : 



Experiment 21. — The same specimen employed in the 

 preceding experiment was taken, and the inside of the canal 

 surrounded by the First Radials freely painted with nitric acid, 

 until the pentagonal comn)issure was exposed and destroyed. 

 The animal was then returned to the water and left for half an 

 hour on its oral face, where it remained without any attempt 

 to right itself or to swim. The arms were then strongly nipped 



