ME. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSA. 
283 
been practicable to get the electrodes adjusted accurately upon the various minute 
histological tracts of tissue, and at the same time to get them within the field of the 
microscope. The power I used was one-inch Ross, and the strength of the induction- 
shock was of course regulated in the ordinary way. 
The following are the results yielded by this method of investigating the distribution 
of excitable tracts in Sarsia ; and it is interesting to observe how uniformly they 
coincide with the results obtained by section. I should like to observe, however, that 
some of the observations are attended with considerable difficulty (arising from escape 
of the current, conductivity of the gelatinous tissue, &c.) ; so that, although I have 
spent a great deal of time and trouble over this part of the inquiry, I desire it to be 
understood that I intend to confirm these observations more extensively next year. 
The apex of the swimming-bell of Sarsia is much the least excitable portion of the 
animal ; and from this point downwards to the margin there is a beautiful and uninter- 
rupted progression of excitability, the latter being greatest of all when the electrodes 
are placed upon the string of cells described by^ Agassiz as nerve-cells. The actual 
proportions in one average case were as follow : — 
(i) Electrodes on apex of inner bell. No contraction on strongest shock. 
(ii) Electrodes a quarter of an inch from margin. Bell first contracted with secon- 
dary coil at 2 centims. 
(iii) Electrodes one eighth of an inch from margin. Bell first contracted with 
secondary coil at 8 centims. 
(iv) Electrodes one sixteenth of an inch from margin. Bell first contracted with 
secondary coil at 5 centims. 
(v) Electrodes on the marginal canal. Bell first contracted with secondary coil at 9 
centims. 
Of the marginal tract of excitable tissue, the degree of excitability differs slightly in 
different parts. It is least when the electrodes are placed midway between the two 
eye-specks ; it is somewhat greater when an eye-speck is included between the 
electrodes. The excitability is again slightly higher when one electrode is placed in an 
eye-speck and the other outside it. Still greater is the excitability when both electrodes 
are put into one eye-speck ; and the excitability is greatest of all when care is taken to 
place both electrodes in that half of the eye-speck which is above the pigment-spot. 
In these remarks of course I wish it to be understood that the excitability is greater in 
the parts named than in other parts, when an equal amount of tissue is included 
between the electrodes in the case of both experiments constituting any comparative 
observation. The following are the ratios in an average case : — 
f (i) Electrodes between two eye-specks. Bell first contracted at 6|- centims. 
1 (ii) Electrodes with one eye-speck between them. Bell first contracted at 7|- centims. 
f (iii) Electrodes with one eye-speck between them. Bell first contracted at 7 centims. 
1 (iv) One electrode in an eye-speck. Bell first contracted at 8 centims. 
f (v) Both electrodes just outside an eye-speck. Bell first contracted at 8 centims. 
1 (vi) Both electrodes in the eye-speck. Bell first contracted at 10 centims. 
