124 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
persistent contraction, becattse a spasmodic con- 
traction, besides being of unusual strength, is also 
of unusual duration; that is to say, while an 
ordinary systolic movement only lasts a short time, 
a spasm lasts from six to ten seconds or more, and 
this whether it occurs in a large or in a small piece 
of tissue. Again, the diastolic movements differ 
very much in the case of an ordinary locomotor 
contraction and in that of a spasm; for while in 
the former case the process of relaxation is rapid 
even to suddenness, in the latter it is exceedingly 
prolonged and gradual, occupying some four or five 
seconds in its execution, and, from its slow but 
continuous nature, presenting a graceful appearance. 
Lastly, the difference between the two kinds of 
contraction is shown by the fact that, while a spasm 
is gradually passing off the ordinary rhythmical 
contractions may often be seen to be superimposed 
on it—both kinds of contraction being thus present 
in the same tissue at the same time. 
Now the point with which we shall be especially 
concerned is, that it is only stimulation of certain 
parts of the organism which has the effect of throw- 
ing it into a spasm. These parts are the margin 
(including the tentacles) and the courses of the 
four radial tubes (including the manubrium, which 
in this species is spread over the radial tubes). 
This limitation, however, is not invariable; for I 
have often seen individuals of this species respond 
with a spasm to irritation of the general contractile 
tissue. Nevertheless, such response to such stimu- 
lation in the case of this species is exceptional— 
