512 A. MILNES MARSHALL. 



additional commuQications between the right and left axial 

 cords. 



In the arms the axial cords lie in tubular channels per- 

 forating the calcareous joints (figs. 1 and 2). Each cord gives 

 off alternately right and left stout branches, which enter the 

 pinnules (fig. 2, c), in which their relations are the same as in 

 the arms themselves. Besides these, finer branches are given 

 off, both from the axial cords themselves and from the pinnule 

 branches, which, passing through the calcareous joints, can be 

 traced into very intimate relation with the muscles moving 

 the arm-joints on one another, with the tentacles and the 

 crescentic leaflets bordering the ambulacral groove, and with 

 the tegumentary covering of the arms generally. 



Histologically, the central capsule and the various cords in 

 connection with it consist principally of very delicate fibrils, 

 arranged for the most part longitudinally, and having inter- 

 spersed among them very small nucleate cells, both the cells 

 and fibrils closely resembling those of the subepithelial bands 

 of the ambulacral grooves. Other fibres, of more irregular size 

 and distribution, which traverse both the capsule and cords in 

 various directions, appear to be of the nature of connective 

 tissue, and to correspond to the vertical strands in the subepi- 

 thelial bands. Externally both the capsule and the cords are 

 invested by a layer of cells, which are much larger than the 

 small ones found in the substance of the cords, stain deeply, 

 and give off branching processes which are in very close rela- 

 tion with the reticulum forming the organic basis of the skeletal 

 parts. This external layer of cells appear to me to be a mere 

 investment of the cords and to be no part of their real 

 substance. 



The pinnules of each arm arise alternately from the right 

 and left sides, each of the brachials except the first bearing 

 one pinnule. The structure of the pinnules is, with certain 

 exceptions, the same as that of the arms, each having an ambu- 

 lacral groove, subepithelial band, tentacles, ambulacral, sub- 

 tentacular, genital and coeliac canals, a branch of the axial 

 cord, &c. ; the genital rachis, however, which is only a slender 



