STAR-FISH AND SEA-URCHINS. ol7 
nerve-ring is that of entirely destroying the ability 
to perform this manceuvre in the case of the majority 
of specimens ; nevertheless about one in ten continue 
able to perform it. Again, if an Echinus is divided 
into two hemispheres by an incision carried from pole 
to pole through any meridian, the two hemispheres 
will live for days, crawling about in the same 
manner as entire animals; if their ocular plates 
are not injured, they seek the light, and when 
inverted they right themselves. The same observa- 
tions apply to smaller segments, and even to single 
detached rows of ambulacral feet. The latter are, 
of course, analogous to the single detached rays of 
a Star-fish, so far as the system of ambulacral feet 
is concerned ; but, looking to the more complicated 
apparatus of locomotion (spines and pedicellarie), 
as well as to the rigid consistence and awkward 
shape of the segment—standing erect, mstead of 
lying flat—the appearance presented by such a seg- 
ment in locomotion is much more curious, if not 
surprising, than that presented by the analogous 
part of a Star-fish under similar circumstances. It 
is still more surprising that such a fifth-part seg- 
ment of an Echinus will, when propped up on its 
ab-oral pole (Fig. 62), right itself (Fig. 63) after 
the manner of larger segments or entire animals. 
They, however, experience more difficulty in doing 
so, and very often, or indeed generally, fail to 
complete the manceuvre. 
On the whole, then, we may conclude that the 
nervous system of an Echinus consists (1) of an 
external plexus which serves to unite all the feet, 
