300 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
logically separated from the rest of the organism. 
If the two nerve-divisions are so placed as to in- 
clude two adjacent rays—zwze. if one cut is on one 
side of a ray and the other on the further side of 
an adjacent ray—then these two rays remain in 
physiological continuity with one another, although 
they suffer physiological separation from the other 
three. When a Brittle-star is completely divided 
into two portions, one portion having two arms and 
the other three, both portions begin actively to turn 
over on their backs, again upon their faces, again 
upon their backs, and so on alternately for an in- 
definite number of times. These movements arise 
from the rays, under the influence of stimulation 
caused by the section, seeking to perform their 
natural movements of leaping, which however end, 
on account of the weight of the other rays being 
absent, in turning themselves over. An entire 
Brittle-star when placed on its back after division 
of its nerve-ring is not able to right itself, owing to 
the destruction of co-ordination among its rays. 
Astropecten, under similar circumstances, at first 
bends its rays about in various ways, with a pre- 
ponderant disposition to the tulip form, and keeps 
its ambulacral feet in active movement. But after 
half an hour, or an hour, the feet generally become 
retracted and the rays nearly motionless—-the 
animal, like a Brittle-star, remaining permanently 
on its back. In this, as in other species, the effect 
of dividing the nerve-ring on either side of a ray 
is that of destroying its physiological connection 
with the rest of the animal, the feet in that ray, 
