VIII EGHINODERMATA— NERVOUS SYSTEM 459 



radial nerve trunk. The radial nerve trunk of the su})erficial system 

 and the paired nerve bands of the deeper nervous system are separated 

 merely by a thin structureless membrane. The superficial circular 

 nerve is similarly accompanied by a band of the deeper nervous 

 system. 



The paired bands become thickened at regular intervals, simul- 

 taneously with the radial nerve trunks of the superficial system. 

 Between the consecutive swellings of this latter, however, they are 

 extremely thin. The deeper nerve ring is much thicker in its inter- 

 radial than in its radial portions. 



In each interradius two nerves are given off" by the deeper nerve 

 ring; these break up into several branches and innervate the inter- 

 radial muscles of the oral skeleton. 



The paired radial bands of the deeper system give rise, at points 

 which regularly alternate with the tentacle and peripheral nerves, to 

 nerves which, first penetrating the cavity of the pseudohsemal canal, 

 ascend apically, enter the vertebral ossicles, ramify in tliem, and inner- 

 vate the intervertebral muscles. The vertebral nerve of one side 

 always innervates the dorsal and ventral intei'vertebral muscles of the 

 same side of the arm, which counteract the homologous muscles of the 

 opposite side. 



3. Eehinoidea. 



Only those Echinoids which are provided with a masticatory 

 apparatus have a deeper nervous system ; this bears out the very 

 probable assumption that this system innervates the masticatory 

 muscles. It consists of five lamellpe consisting of nerve cells and 

 fibres, which are in close contact with the radial portions of the super- 

 ficial oesophageal ring and the points of departure of the radial nerves. 

 Each lamella gives off a pair of large nerves. These ascend along the 

 edges of the five jaws, then ramify, and most probably, as already 

 stated, innervate the jaw muscles. 



4. Holothupioidea (Fig. 352, p. 409). 



The deeper nervous system is only developed on the radial nerve 

 trunks, which it covers on their inner sides in the form, in each case, 

 of a single thin band, consisting of nerve cells and longitudinal fibres. 

 The nerves which proceed from these radial bands seem to serve 

 principally for the innervation of the dermo-muscular tube. 



C. The Apical or Aboral Nervous System. 



The apical nervous system of the Asteroidea, Eehinoidea, and 

 Ophiuroidea, has already been snfliciently described at the beginning of 

 this section, p. 454. That of the Crinoidea, however, requii'es some 

 further elucidation. 



