480 



Mr. G. J. Romanes on the 



[Jan. 11^ 



tentacular waves varies in diferent cases ; but very often the tolerance 

 o£ tentacular waves towards section is quite as remarkable as is that o£ 

 contractile waves described in my former paper. Or, adopting our pre- 

 vious termiuology, section proves that in Aurelia aurita physiological 

 harmony is as difficult to destroy as is physiological continuity. And 

 this fact I regard as the most noteworthy of all the facts to which the 

 present investigation has led — if, indeed, it is not the most startling and 

 significant that has yet been brought to light in the whole range of Inver- 

 tebrate physiology. 



That the fact before us cannot be explained by supposing, with Klei- 

 nenberg, that the functions of nerve and muscle are blended in the same 

 cells, would seem to be conclusively shown by the additional facts, that in 

 some cases section blocks the stimulus-wave before it blocks the contrac- 

 tile wave, while in other cases the reverse is true. The only other hypo- 

 thesis open to us seems to be that of a rudimentary nervous plexus, the 

 constituent elements of which must be supposed capable of vicarious 

 action in almost any degree. This, it will be remembered, was the hypo- 

 thesis to which I inclined last year as an explanation of all the facts 

 relating to the passage of contractile waves ; and the hypothesis is now 

 greatly strengthened by the additional facts just stated with regard to 

 the passage of stimulus-waves : for while the maintenance of physio- 

 logical continuity is a function of muscle, the maintenance of physio- 

 logical harmony is a function of ner^e. It therefore seems to me that 

 at the stage the inquiry has now reached there is no conceivable alterna- 

 tive between accepting this h}^3othesis and abandoning the whole subject 

 as inexplicable. 



Moreover this hypothesis is supported by the fact, that explorations of 

 the swimming-organ of Aurelia with graduated stimuli reveals the pre- 

 sence of excitable tracts pervading the tissues in all dii'ections ; but I 

 cannot here enter into particulars. Lastly, it must be stated that we 

 are led up to this hypothesis by degrees. Sarsia is the most highly 

 organized of the Medusae which I have examined ; and here I have found 

 that the supposed plexus is so far differentiated, that vicarious action on 

 the part of its constituent elements is usually possible in but a low 

 degree. On the other hand, Aurelia is the least highly organized species 

 which I have examined ; and here the supposed plexus is so slightly dif- 

 ferentiated, that vicarious action on the part of its constituent elements 

 is possible in a very high degree. Lastly, the discophorous species of 

 naked-eyed Medusae stand midway between Sarsia and Aurelia in respect 

 of the degree in which integration of their organs has proceeded ; and in 

 them the vicarious action of the supposed nerve-plexus also occupies an 

 intermediate position, as we shall see while considering an interesting 

 series of facts to the discussion of which I will now pass. 



§ 3. Character of the E.vcitahle Tissues o/Tiaropsis indicans. — Tiarojpsis 

 indicans is a bowl-shaped species of naked-eyed Medusa, having a manu- 



