Pomona College, Claremont, California 



51 



smaller at the region of the terminal tentacle in the ocular plate. 

 This might suggest something as to the nerve tracts or bundles of 

 fibers and gives an indication at least that fibers may convey im- 

 pulses at different distances such as in the central nervous system 

 of vertebrates. 



The deep radial and circum-oral strands of sea-urchins are 

 poorly shown in section. Only a few cells scattered along the 

 inner margin of the fibrous region give an indication of this poorly 

 developed system. 



In the sand-dollar, Dendraster excentricus some variation in 

 form is suggestive of value in comparison with other forms. 



The righting reactions in sea-urchins are carried out with 

 greater difficulty than in starfish and only the fresher or more vig- 

 orous individuals are capable of the reaction. 



Fig. 29. Nervous system of Sea-urchin. A. Diagram of nervous system of 

 sea-urchin showing in various ways the superficial and deep nerv- 

 ous system by having the superficial system cut away on part of 

 the two radial nerves at the left. Branches to the tube-feet shown 

 in the central of the three ambulacral areas. Nerves to the bases 

 of the spines show on the right. Superficial nerve plexus show in 

 the center. B. Diagram of the nervous system from the aboral 

 pole, showing the nerve connections at the genital openings and the 

 ends of the radial nerves at the five ocular plates. C. Diagram of 

 cross section of nervous system having branches to a spine and 

 a tube-foot after Delage and Herouard. 



Although the eye-spots of sea-urchins are not well developed 

 they seem to avoid light and seek darker corners and sheltered 

 places. One form which has no eye-spots seems to avoid the light. 

 A sudden shadow falling on it causes it to direct its spine to the 



