VIII ECHmODEBMATA— NERVOUS SYSTEM 461 



D. The third Nervous System of the Crinoidea. 



Besides the superficial oral and the apical systems, the Crinoids 

 have auother nervous system. This is developed on the oral side of 

 the disc and of the arms, and is subepithelial in position. It consists 

 of the following principal parts : (1) a nerve ring eneireling' the 

 oesophagus close to the mouth ; and (2) five pairs of brachial 

 nerves. The two nerves of each brachial pair run down the arm 

 longitudinally at the sides of the radial water vascular canal (Fig. 

 356, 4, p. -1:13). They are found on the branches of the first, second, 

 and other orders. Their condition at the points where the arms 

 divide is not yet known. 



In addition to the five pairs of brachial nerves, the oesophageal ring 

 gives rise, in each interradius, to two nerves which branch freely and 

 run to the bands and mesenteries which traverse the body cavity, 

 giving off branches also to the tegmen calycis. 



Lateral branches of the paired brachial nerves innervate the mus- 

 culature of the water vascular and tentacle canals which run in the 

 arm ; they also ascend in the tentacles to innervate their sensory 

 papillse. 



This third nervous system of the Crinoids is connected with the 

 apical system by means of branches in the following manner : — 



1. The two brachial nerves of each arm send off branches alter- 

 nately (first from the right nerve, theu from the left) towards the 

 apical side of the arm. Each of these unites with one of the pair of 

 nerves which descend orally from the apical nerve trunk within the 

 interior of the brachial joint (Fig. 356, 10, ]}. 413). 



2. Certain lateral branches of the pair of nerves which run out 

 from the oesophageal ring in each interradius seem to run along the 

 body wall apically, and to unite with lateral branches of the apical 

 nerve trunks which come from the nerve envelope of the chambered 

 sinus. 



Although recent investigators incline to tlie opinion that this third nervous 

 system in the Crinoids has no representative in the other Echinoderins, the question 

 may here be raised whether it does not correspond with a deeper oral nervous system. 

 If we imagine the deeper oral nervous system of an OxiMurid or an Asteroid to have 

 become severed from the superficial system and to have shifted further below the 

 surface, we have a system showing considerable lesemblance to the third system of 

 the Crinoids. Because no connection between the deeper oral system and the apical 

 system has yet been demonstrated in other Echinoderms, we have no riglit, considering 

 the difficulty of investigation on this point, to conclude that no such connection 

 exists. It is difficult to believe that there are tliree completely independent nervous 

 systems in the Echinoderm body. 



