MONOGRAPH OF THE EXISTING CRINOIDS. 331 



this pair of branches divides and enters the arms, subsequently dividing in such 

 a way that a similar pair of nerve cords is always found in the ventral perisome 

 above the water vessel which traverses each arm and pinnule. In addition to these, 

 Hamann usually found branches, commonly two, between each two radial water 

 vessels leading outward and ramifying into the skin in every direction as far as 

 the border of the disk and giving off strong nervules which ramify in the connective 

 tissue of the mesenteries, bands, and septa traversing the body cavity, where they 

 are met with everywhere. Derivatives from this circumoral pentagon innervate 

 the epithelium of the madreporic or calicular pores. 



From the two ventral nerve cords accompanying the water vessel of the arms 

 and pinnules arise a number of fine branches which run to the musculature of the 

 water vessels and their ramifications in the tentacles, and a branch which runs 

 along the connective tissue of each tentacle to its tip. The sense papillae of the 

 tentacles are innervated by branches from this ventral nervous system, the fibrillse 

 from the epithelial nerve plexus being only of secondary importance. 



Throughout the length of the arms and pinnules an intimate connection is 

 maintained between this system of paired ventral cords and the axial cord of the 

 dorsal portion, the latter sending out in the middle of each brachial at the point 

 of origin of the pinnule nerve and opposite it connectives which unite it alternately 

 with the right and left cords of the ventral system ; the length of these connectives 

 in Antedon mediterranea is 0.82 mm. ; furthermore, the peripheral branches of the 

 two systems are in union with each other. 



These nerves and their branches are all compact and solid in their structure, 

 like those of the dorsal nervous system. Large bipolar and multipolar ganglion 

 cells resembling those of the dorsal nervous system occur, for the most part lying 

 peripherally, with some scattered through the fibrillar mass, especially along the 

 two main branches in the arms. 



INNERVATION OF THE TENTACLES. 



The epithelium on the side of the tentacles toward the ambulacral groove is 

 greatly thickened, equaling in thickness that of the ambulacral groove itself. 

 According to Hamann, its height in Heliometra glacialis is 0.08 mm. It is com- 

 posed of epithelial sense cells intermixed with supporting cells. 



This epithelium, together with its papillae, is innervated by a lateral branch 

 from the epithelial nerve of the adjacent ambulacral groove, which can be traced 

 nearly to the tip of the tentacle. 



The epithelium on the outer side of the tentacle is innervated by a lateral 

 branch from the adjacent longitudinal cord of the ventral nervous system, which 

 may be followed to the tentacle tip. 



XEURILEMMA. 



In Heliometra and in Antedon the only structure which might possibly be 

 interpreted as a neurilemma is the uncalcified connective tissue layer about the 

 axial nerve cord of the arms in which wandering cells occur here and there in 



