50 Journal of Entomology and Zoology 



ECHINOIDEA 



The nervous system of sea-urchins may be compared with that 

 of starfish more easily than with that of other forms. 



The nerves corresponding to the superficial radial and circum- 

 oral nerves are more deeply placed than in starfish and as in star- 

 fish are the most obvious parts of the nervous system. An epi- 

 neural space or tube on the outer side of the nervous band forms 

 the so-called "epineural cavity" or nerve tube, as interpreted by 

 Phouho, '87, and others. The radial and circum-oral sinus follows 

 the nervous system on the inside. 



The superficial radial system follows down the inside of the 

 shell in the center of the ambulacral area and these five strands 

 join with the circum-oral ring about the mouth opening. 



From the nerve ring between the junctions of the five radial 

 nerves are branches to the intestine which go to make up the intes- 

 tinal plexus.^ Nerves run out laterally from the radial nerves to 

 the tube-feet and also to the surface, to the bases of the spines and 

 to the ganglia at the bases of the spines. The radial nerves end 

 in the terminal tentacles through holes in the shell about the anal 

 region. It is by way of these openings, according to Phouho, that 

 the radial nerves contribute to the superficial nerve plexus just 

 outside the test of the sea-urchin. The deep radial nervous system 

 is but poorly represented, so little of it is present closely applied 

 to the superficial radial and circum-oral that it can hardly be recog- 

 nized apart from it. 



According to some, a pentagonal area of aboral nerves sur- 

 rounds the anus and communicates with the genital organs and 

 with the external superficial system by means of fine fibers from 

 the radial nerves near their termination in the terminal tentacle. 

 It is quite probable that the superficial system communicates with 

 that of the shell at the aboral end not only through the so-called 

 ocular openings but also through the genital openings in the shell. 

 The surface of the body, the spines and the tube-feet, are all 

 organs of the tactile sense at least. 



The so-called eye-spots at the terminal tentacle in the five ocu- 

 lar plates contains pigment and may have some sensitiveness to 

 light, but it is not like the eye-spots of starfish and may indeed not 

 be in any sense an eye-spot. 



The chief parts of the system such as the radial and circum- 

 oral nerve bands are composed of about the same parts as in the 

 starfish. In smaller and perhaps younger specimens the outer 

 nuclear layer is thicker in proportion. Nerve cells are bi- and 

 multipolar. In some cases at least multipolar cells are found well 

 within the fibrous area of the strand. Many of the outer cells are 

 probably as in other echinoideans supportive in function. The 

 radial bands are thicker at the oral region and become somewhat 



