124 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



persistent contraction, becaitae a spasmodic con- 

 traction, besides beinof of unusual strencrth, 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. Ag^ain, 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 snch stimu- 

 lation in the case of this species is exceptional — 



