330 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



of these branches from the axial cord. In the regenerating animal the nerve 

 elements are mostly in continuity with the processes of the pear-shaped cells, but 

 in the normal adult it appears more as if the fibrillse ran upon the cells and 

 surrounded them. 



In Heliometra. glacialis Hamann was able to trace the single unpaired nerves 

 which unite the dorsal axial cord of the arm with the paired longitudinal nerves, 

 one of which runs on either side of the water vessel. These nerves run undivided 

 in a nearly straight line. Like the other nerve branches, they are composed of 

 parallel unbranched fibrillse the multipolar ganglion cells of which lie peripherally 

 or more or less within the nerve mass. 



The structure of the axial cord of the arms and of its derivatives appears 

 to be practically identical in Comatella nigra, Comanthus parvicirra, Tropiometra 

 picta, Antedon bifida, A. mediterranea, A. adriatica, Heliometra glacialis, and 

 Isocrinus decorus. Most of the finer work has been done on Heliometra glacialis 

 on account of its large size. 



WANDERING CELLS. 



In intimate connection with the elements of the mesodermal nervous system 

 and of extreme importance in regeneration are the large wandering cells, which 

 were first differentiated from ganglion cells by Hamann, and later described in 

 detail by Reichensperger. 



These wandering cells are very much more numerous in Antedon and Isocrinus 

 than in any of the Comasteridse. In Antedon they often form a complete sheath 

 about the central organ in the centrodorsal, as well as about all the nerve cords 

 radiating therefrom and their branches, occurring as isolated cells to the outermost 

 fine twigs. They also are found, though usually somewhat less abundantly, in 

 connection with the ventral nerve cords. 



The wandering cells occur in every conceivable form; they may be almost 

 spherical, elongate oval, or elongate to wormlike; sometimes they have three or 

 four processes. In the resting condition their contents are either homogeneous, 

 or they show in the feebly stained cytoplasm masses of fine darker and lighter 

 irregular granules and rods. The nucleus is not very large, stains moderately, 

 and almost never exhibits details. 



The size of the wandering cells can be only approximately given; according 

 to the amount of elongation they vary from 34 JA by 2 jji to 52 [jt. by 1.5 jj,. 



VENTRAL NERVOUS SYSTEM. 



The ventral nervous system, which is intimately connected with the dorsal 

 but entirely distinct from the epithelial nervous system, centers in a pentagonal 

 circumoral nerve ring, oval in section and about 0.03 mm. in dorsoventral diameter, 

 which lies in the cutis on the same level as the circumoral canal of the water 

 vascular system. 



This nerve ring gives off a very large number of branches. Radiating inward 

 are branches leading to and entering the tentacles ; outwardly on either side of the 

 radial water tubes there runs a longitudinal branch, 0.02 mm. in diameter, and 



