456 



CHAP. 



run through it. At regular segmental intervals, about on a level with 

 the consecutive pairs of tentacles, these trunks show distinct swellings, 

 from which most of the nerves originate. In this way the radial 

 nerves superficially resemble the ventral ganglionic chain of many 

 Annulata and Arthropoda. 



The central portion of the deeper oral nervous system is so closely 

 applied to the corresponding portion of the superficial nervous sys- 

 tem that the two can only be distinguished from one another by means 



is 



Fio. 372. Nervous system of an Ophiurid (Ophiothrix fragilis) (after Cuenot). Part of the 

 disc and the base of an arm. 1, Peripheral brachial nerve ; 2, tentacle nerve ; 3, nerve to the 

 muscles between the vertebral ossicles ; 4, radial nerve trunk ; ">, ganglion at the base of a spine ; 

 6, marginal nerve of the disc ; 7, bursal aperture ; 8, nervus lateralis ; >, gonad projecting into the 

 bursa ; 10, Polian vesicle ; 11, interradial nerve ; 12, nerve of the musculus interradialis aboralis 

 (Simroth); 13, nerve ring; 14, dental nerve; 15, enterocoelic nerve ring; 16, spine; I, first oral 

 tentacle ; II, second oral tentacle ; III-VIII, tentacles of anus. 



of careful microscopic examination, e.g. of transverse sections. In the 

 following description, however, we shall keep the two systems entirely 

 distinct from one another. 



Nerves of the cesophageal ring. A large number of nerves arise from the 

 oasophageal ring, and ramify in the connective tissue layer of the intestinal wall. 

 Further, at each of the points where a radial nerve trunk joins the cesophageal ring, 

 the latter gives off two nerves, which run to the bases of the first pair of oral tube- 

 feet. Each of them there forms a circular ganglion, almost completely surrounding 



