NERVOUS SYSTEM. INVERTEBRATA. 3 



oesophagus, and of five radial cords. Each cord arises from 

 the ring at a point opposite one of the radii and passes 

 down the arm between the vertebral ossicles and the ven- 

 tral plates (fig. 2) . A cavity (epineural canal) lies between 



A.V 



T. F. 



Diagrammatic transverse section of the Arm of Ophiocoma echinata. x 40. 



A.V. Ambiilacral vessel. E.G. Epineural canal. M. Intervertebral 



muscle. P. Median partition of epineural canal. P.V. Pseud- 



hsemal vessel. R.C. Radial cord. T.F. Tube-foot. V. Vertebral 

 ossicle. V.P. Ventral plate. 



the body-wall and the superficial surface of the oral ring 

 and radial cords ; it is usually single, but in Ophiocoma a 

 partition of connective tissue divides it longitudinally into 

 two separate channels. 



The superficial oral system is to a large extent sensory 

 in function ; it innervates the entire body surface, the 

 ambulacra, the mouth, and alimentary canal. The deep 

 oral system (not distinguishable in the specimen) forms a 

 thin layer of nervous tissue upon the deep surface of the 

 superficial system, separated from it by a delicate layer of 

 connective tissue. Upon its deep surface lie the pseud- 

 haemal and ambulacral vessels. The deep oral system is 

 the motor centre for the intervertebral muscles of the arms, 

 and probably also gives off fibres that accompany the peri- 

 pheral and ambulacral nerves of the superficial system. 



The oral ring and proximal parts of the radial cords are 

 shown in an isolated state in the upper specimen, and in 



B2 



