140 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



INTERNAL CAVITY OF THE CALYX. 



A. The Chambered Organ and the Axial Canals. 



A striking feature in the organization of the Comatulge is the quinquelo- 

 cular organ, situated in the cavity of the centrodorsal, and placed at right 

 angles with the central axis. This organ was first noticed by Hensinger, 

 who in 1828 described it as the central organ of the blood vascular system. 

 Mliller also took it to be a heart-like organ in connection with a system 

 of membranous tubes. Dr. W. B. Carpenter regarded the membranous 

 tubes of Mliller as solid fibrillar cords, proceeding from a similarly con- 

 stituted envelope around the chambered organ, and he came to the con- 

 clusion, that this fibrillar sheath, and the cords proceeding from it, constitute 

 the central nervous system of the Comatulae. This was afterwards confirmed 

 by experimental evidence, and is now generally admitted by zoologists. 



The organ in question is a sac, divided into five radial compartments, 

 enclosed by a thick envelope in connection with the axial cords. From the 

 dorsal surface of this envelope processes are given off to the cirri, and from 

 its margin arise interradially five short primary cords, which, passing up- 

 wards and outwards, bifurcate into right and left branches between the 

 centrodorsal and radials. The ten secondary cords diverging from one 

 another, enter the substance of the radials, and either unite in pairs, the 

 right branch from one interradial meeting the left branch from the adjoining 

 one (Figs. 3 and 4), or the two branches, as in Encrmus liliiformis (Fig. 5), 

 without touching each other, proceed on separately to the costals. On 

 reaching the first axillaries the two cords open out into two branches, right 

 and left, and after traversing the plates, enter the right and left arms, 

 respectively. In addition to the above connections, there is a circular or 

 pentangular commissure, which, immediately after entering the radials, con- 

 nects the various branches among themselves, and additional connections 

 between the branches within the axillaries supply the arms (Figs. 3 to 4). 

 The axial cords along the arms lie in tubular channels piercing the calcareous 

 part of the various arm joints, each cord giving off alternately right and 

 left branches, which enter the pinnules. 



Chambered organs have been observed also in Stalked Crinoids, but the 

 position is not quite the same as in the Comatulge. While in the latter the 



