26 



5. The nervous System. 



In the nervous system we find again the same radial arrangement as has been al- 

 ready mentioned in reference to the ambulacral system. In the main features it corresponds 

 completely with what we know in other Asteridse. The chief parts, or the central parts, are 

 formed by the ambulacral nerves belonging to the arms, or as we may call them the ambu- 

 lacral brains. They have each (Tab. VI, fig. 4, 5) the form of a narrow band lying quite 

 superficially, and extending along the middle of the ambulacral furrow in the whole length 

 of the arm. In the middle this band forms a distinctly elevated ridge, arching over a narrow 

 longitudinal canal or sinus which is divided by a medial septum (a) in two compartments. 

 During its course it sends out on each side fine converging nerves to the water-feet, and in 

 the spaces between the latter it expands on each side, insensibly merging in the cuti- 

 cular lining. It terminates at the point of the arm in a somewhat claviform enlargement 

 (fig. 12 b) which lies at the base of a peculiar organ of sense herein-after more particularly 

 described. The whole nervous band is of a light yellowish color and is particularly brittle 

 and fragile, lying so closely enveloped in the thin membrane which lines the ambulacral 

 furrow, that it can only with extreme difficulty be dissected out and isolated from the adja- 

 cent parts. On removing the water-feet however, it may be distinctly perceived along the 

 whole length of the ambulacral furrow. These nervous bands belonging to the arms are 

 continued immediately inwards on the underside of the disc, where they likewise occupy 

 the middle of the ambulacral furrow, and when the water-feet are removed, appear very 

 distinctly (Tab. VI, fig. 1 b). At the interior extremity of the ambulacral furrows, all 

 the ambulacral nerves are connected by a somewhat broader band-like circular com- 

 missure (ibid a) situated close in against the bucal ring at its junction with the bucal 

 membrane. This commissure lies, like the ambulacral nervous band itself, quite superficially 

 extending along the lower side of the circular rim which projects from the inner wall of the 

 bucal ring, and it is, only at the end of the interradial spaces, partially covered over by 

 the edges of the inner adambulacral plates with their diverging spines. Like the ambulacral 

 nerves this commissure also seems to arch over a narrow circular sinus, which stands in con- 

 nection with the above-mentioned radial canals running along the ventral furrows of the arms. 



6. Organs of sense. 



Under this category we must class a peculiar organ which is situated at the extreme 

 point of all the arms. The organ exists there, as well in the previously observed form B. 



