163 REPORT— 1876. 



swiinmiug-organ be very gently iri'itated, the resulting contractile '^vaye does not 

 start from the immediate seat of irritation, but from the ganglion which still 

 remains in situ. 



But this allusion to a " contractile wave " renders it necessary to state that all the 

 conti'actile motions of the Meduste (whether due to ganglionic or to artificial 

 stimulation) may be seen to be of the natme of contractile waves which spread 

 from the i^oint of stimulation as from a centre. The rate at which they travel 

 varies greath' in diflerent species, and in the same species imder dilterent conditions 

 of temperature, &c. The author has made an elaborate series of experiments by 

 section, with the view of ascertaining whether these contractile waves are merely 

 muscle- waves or depend for their passage upon the presence of rudimentary nerves. 

 He finds that the tissue will endure almost any severity of overlapping SL^ctions 

 without suflering loss of its physiological continuity — the contractile waves still con- 

 tinumg to zigzag back and fore among the overlapping cuts. Similarly v/ith 

 another form of section, which consists in carrying a cut round and round the 

 swimming-disk in the form of a spiral, the Medusae being thus converted into the 

 form of a ribbon. In such a form of section the author has repeatedly seen con- 

 tractile waves passing freely from end to end of a ribbon-shaped strip of tissue 

 measuring only an inch across and more than a yai-d in length. He was therefore 

 at first inclined to regard these contractile waves as merely muscle-waves. Never- 

 theless there is likewise an important body of evidence to be adduced in favour of 

 a nervous plexus. In particular, if the spiral mode of section be carried on suffi- 

 ciently fai', a point is, sooner or later, sure to come at which i\\e contractile waves 

 cease to pass forward : they become blocked at that point, and this ahcays with 

 t/reat stidcknness. Moreover, the point at which such blocking of the waves takes 

 place is extremely variable in diflerent individuals of the same species. Lastly, 

 the fact tlir.t reflex action has been proved to occur, shows that these excitable 

 tissues are pervaded by tracts which present the distinguishing function of nei-ve, 

 viz. the conveying of impressions to a distance. And it is of the first importance 

 to obser\-e that this fimction is quite as difficult to destroy by the introduction of 

 overlapping or of spiral cuts as is the function on which the passage of contractile 

 waves depends. lu other words, reflex action continues to take place through 

 forms of sectiou as severe as those through which contractile waves continue to 



!)ass. And tliis f;ict the author considers the most important that has as yet been 

 nought to ligbt in the whole range of invertebrate physiology ; for he regards it as 

 evidence that in these primitive nervo-muscular tissues the conductile or nervous 

 element becomes diflerentiated from the contractile or muscular element in such 

 a way that vicarious action is permitted to take place to any extent among the 

 incipient conductile elements. And in .striking confirmation of this view another 

 series of observations may here be mentioned. 



Tiaropsis inclicans is a bowl-shaped species of naked-eyed Medusfe, to which the 

 author has assigned this name in reference to a highly interesting function that is 

 manifested b_v its manubrium. This fuuction consists in the organ localizing, with 

 the utmost precision, any point of irritation which is situated in the bell. For 

 iustance, if any point in the irritable surface of the bell be pricked with a needle, 

 the manubrium moves over towards that point and applies its tapered extremity 

 to the exact spot where the prick has been inflicted. But now, this unerring pre- 

 cision with wliich the manubrium indicates a seat of iriitatiou in the bell may be 

 completely destro^'ed by introducing a short cut between the base of the manubriiun 

 and the seat of irritation in the bell. The afferent connexions, therefore, on which 

 this localizing function depends are thus shown to be exclusively, or almost exclu- 

 sively, radial. But although under these conditions the manubrium is no longer 

 able to localize the seat of irritation, it nevertheless continues to perceive, so to speak, 

 that irritation is being applied someivherc ; for every time the irritation is applied 

 the manubrium actively dodges about from one part of the bell to another, apply- 

 ing its extremity now at this place and now at that one, as if seeking in vain for 

 the ofl'ending bodv. Now this fact shows that the stimulus, on reaching the point 

 at which the afferent tract is severed, escapes from the severed to the unsevered 

 tracts through the vicarious action of the latter. 



There is another point of interest connected with this apparently reflex action. 



