STAR-FISH AND SEA-URCHINS. 299 
The 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 rays now forming a very marked 
contrast to the instantaneous and vigorous leaping 
movements of escape which are manifested by 
unmutilated specimens. Moreover, unmutilated 
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 viclent 
stimulation of the dise. In other words, the disc is 
entirely severed from all physiological connection 
with the rays. 
If the nerve-ring be divided at two points, one 
on either side of a ray, that ray becomes physio- 
