187;.] 



Locomotor System of Medusa. 



479 



most fundamental quality wherein the function of nerve is distinguished 

 from that of muscle — viz. the power of setting up responsive contrac- 

 tions at a distance from the seat of irritation. In my former paper I 

 described a number of experiments in section, which were devised in 

 order to test the tolerance towards section of physiological continuity : 

 similar experiments, having reference to the tolerance towards section of 

 physiological harmony, are described in the present paper. The experi- 

 ments just detailed in the foregoing paragraphs were really experiments 

 of this kind; but such experiments derive a special interest when con- 

 ducted on the general contractile sheet of swimming-organs. They do 

 so, because it will be remembered that the most interesting questions 

 ^ith which my pre^aous paper was concerned were, first as to the presence 

 of a rudimentary nerve-plexus, and next as to the extent in which, if pre- 

 sent, it was diiferentiated from the muscular element. 



Xow, as stated in my former paper, the contractile tissues of JSarsia 

 will endure very severe forms of section without suffering loss of their 

 physiological continuity ; but I find that, as a rule, their tolerance is not 

 nearly so great as regards maintenance of their physiological harmony ; 

 for in general, though not invariably, the manubrium fails to respond to a 

 stimulus applied to the bell if a cut of a millimetre or two in length 

 intervenes between the base of the manubrium and the seat of irritation. 



§ 2. Character of the Excitable Tissues of Aurelia. — Widely different is 

 the case of Aurelia. As already stated, when a portion of the skimming- 

 organ of this animal is very gently irritated, a contractile wave does not 

 start from the point of u'ritation ; but the passage of a stimulus-wave 

 from that point is proved by the invariable discharge of a gangKon situ- 

 ated at a distance from the latter. It must now be added that the 

 passage of the stimulus-wave admits, in many cases, of being actually 

 seen ; for it is a peculiarity of the innumerable tentacles which fringe 

 the margin of Aurelia, that they are more irritable than the neuro-mus- 

 cular sheet of the smmming-organ. Consequentl}^ a stimulus which is 

 too gentle to cause the latter to respond even when applied directly to 

 its own substance, nevertheless very often causes a response on the part 

 of the former. When this happens, the tentacles all the way along the 

 margin contract in succession, while the swimming-organ remains perfectly 

 motionless. When this tentacular wave reaches a ganghon, it causes the 

 latter to discharge (after ^ sec. or more for the latent period), and so to 

 give rise to a general contraction of the neuro-muscular sheet. This most 

 beautiful expression oi the passage of a wave of stimulation does not 

 occur in all, or even in most, specimens of Aurelia ; and even in those 

 specimens where it does occur, it may be more readily started by stimu- 

 lating some tracts of the neuro-muscular sheet than by stimulating other 

 tracts, although there is no constancy as to the position of these tracts in 

 different individuals. 



Now the severity of section which is required to cause blocking of these 

 VOL. xxy. 2 M 



