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 throwing 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 stimulation in the case of this species is exceptional--the usual response to muscular irritation being an ordinary locomotor contraction, which forms a marked contrast to the tonic spasm that _invariably_ ensues upon stimulation of the margin, and _almost_ invariably upon the stimulation of a radial tube.