STAR-FISII AND SEA-URCHINS. 299 



Tl]c loss of co-ordination between the rays caused 

 by division of the nerve-ring in the disc is rendered 

 most conspicuous in Brittle-stars, from the circum- 

 stance that in locomotion and in righting so much 

 here depends upon co-ordinated muscular contrac- 

 tion of the rays. Thus, for instance, when a Brittle- 

 star has its nerve-ring severed between each ray, 

 an interesting series of events follows. First, there 

 is a long period of. profound shock — spontaneity, 

 and even irritability, being almost suspended, and 

 the rays appearing to be rigid, as if in tetanic 

 spasm. After a time, feeble spontaneity returns — ■ 

 the animal, however, not moving in any determinate 

 direction. Irritability also returns, but only for 

 the rays immediately irritated, stimulation of one 

 ray causing active writhing movements in that 

 ray, but not affecting, or only feebly affecting, the 

 other rays. The animal, therefore, is quite unable 

 to escape from the source of irritation, the aimless 

 movements of the ra3's now forming a very marked 

 contrast to the instantaneous and vigorous leaping 

 movements of escape which are manifested by 

 unmutilated specimens. Moreover, un mutilated 

 specimens will vigorously leap away, not only from 

 stimulation of the rays, but also from that of the 

 disc ; but those with their nerve-ring cut make no 

 attempts to escape, even from the most violent 

 stimulation of the disc. In other words, the disc is 

 entirely severed from all physiological connection 

 witli the rays. 



If the nerve-ring be divided at two points, one 

 on either side of a ray, that ray becomes physio- 



