29 



the commissure. Transverse sections of the brachial nerve 

 cords (PL III., fig. 39) show that their fibrils form five 

 definite strands. Two of these run along the oral aspect 

 of the cord and are in contact in the middle line ; while 

 the remaining three traverse its dorsal face, one being 

 median and the other two lateral. Both the lateral 

 strands contribute fibrils to the pinnular branches. The 

 organic reticulum which forms the bases of the skeletal 

 parts is a definite investing layer around the capsule 

 and the cords. 



Very little is known of the functions of the superficial 

 oral and deeper oral nervous systems. The sub-epithelial 

 bands which underlie the ambulacral grooves so closely 

 resemble, in histological structure and relation to the 

 ambulacral epithelium, the sub-epithelial bauds of the 

 Asteroidea, which are undoubtedly nerves, that there can 

 be little doubt of their nervous character ; but the experi- 

 ments of Carpenter (6), Milnes Marshall (7), and Jickeli (8) 

 show that neither the superficial oral nor the deeper oral 

 system plays more than a very subordinate part in 

 sensation and movement. In all probability the former 

 is concerned with the ambulacral epithelium and the 

 highly sensitive tentacles bordering the ambulacral 

 grooves, structures with which the system is in close 

 anatomical relation. The experiments cited above show 

 that the apical system is the principal one. That the 

 central capsule is a centre from which the complex co- 

 ordinated movements of swimming and righting are 

 controlled is shown by the fact that when this organ is 

 entirely removed the movements cease ; on the other hand, 

 evisceration, with consequent removal of the circum-oral 

 ring of the ambulacral system, has no effect whatever 

 upon them. The commissure lodged in the pentagonal 

 canal of the radial plates co-ordinates the movements of 



