TYPE EGHINODERMA. 



549 



condition described, and no specially-marked differentiation 

 of the tube occurs. 



The nervous system of the Crinoids is characterized by 

 the remarkable development of the aboral portion and by 

 the apparently entire absence of the muscular portion. The 

 epithelial portion consists of a ring and five radial nerves (rm) 

 which pass along the ambulacral grooves and out upon the 

 arms and pinnules, imbedded throughout their entire course 

 in the lower layers of the ectoderm. The aboral system is, 



Fig. 253. — Diagram op the Akrangembnt op the Aboral Nervous 

 System of Antedon (after marshalt,). 

 a = arms. ed = ceiitrodorsal. 



Br = brachial plates. B = radials. 



on the other hand, much more strongly developed, and stands 

 in intimate association with the chambered organ in the walls 

 of which it is imbedded. Its central portion is somewhat 

 complex, as is shown in Fig. 252, but may be said to consist 

 of a ring, or, more properly speaking, of a pentagon from 

 which five strong cords radiate out into the arms, perforating 

 the plates of which these are composed. Both fibres and 

 ganglion-cells enter into the composition of the cords and 

 ring, and a complicated system of commissures exists. From 

 the central portion branches are also sent to the cirri, and 

 probably in stalked forms a branch traverses the central 

 cavity of the stalk, accompanying prolongations of the cavities 



