THE NERVOUS SYSTEM OP ANTEDON ROSACEUS. 267 



(figs. 1, 2, 3 a) in the substance of the brachials or calcareous 

 segments of the arms. 



Besides the connections described above there are certain others 

 which must be noticed. A pentagonal commissure (fig. 3) connects 

 all the branches together immediately after they have entered the 

 First Radial. There is also a further connection in each Third Radial 

 between the branches into which the radial cord divides to supply 

 the two arms of the pair ; this connection, as shown in fig. 3, consists 

 of a transverse commissural band of fibres, and a chiasma formed by 

 two obliquely placed bands which cross one another and furnish 

 additional communications between the right and left axial cords. 



In the arms the axial cords lie in tubular channels perforating 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 con- 

 nection with it consist principally of very delicate fibrils, arranged for 

 the most part longitudinally, and having interspersed among them 

 very small nucleated 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 

 subepithelial 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 relation 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 



