658 KECORD OF CURRENT RESEARCHES RELATING TO 



state tlio frcsli-water Medusa exhibits the power of localizing with its 

 manubrium a seat of stimulation situated in the bell. When a part 

 of the bell is nipped with the forceps, or otherwise irritated, the 

 free end of the manubrium is moved over and applied to the part irri- 

 tated. So far, the movement is precisely similar to that occurring in 

 Tiaropsis indiccms* But there is a curious difference ; for while in 

 T. indicans these movements of localization continue unimpaired after 

 the margin of the bell has been removed, and will be ineffectually 

 attempted even after the bell is almost entirely cut away from 

 its connections with the manubrium, in the fresh-water Medusa 

 the movements cease after the extreme margin of the bell has been 

 removed. For some reason or another the integrity of the margin 

 here seems to be necessary for exciting the manubrium to perform its 

 movements of localization. It is clear that this reason must either be 

 that the margin contains the nerve-centi'es which preside over these 

 localizing movements of the manubrium, or, much more probably, that 

 it contains some peripheral nervous structures which are alone capable 

 of transmitting to the manubrium a stimulus adequate to evoke the 

 movements of localization. In its unmutilated state this Medusa is at 

 intervals perpetually applying the extremity of its manubrium to one 

 part or another of the margin of the bell, the part of the margin 

 touched always bending in to meet the approaching extremity of the 

 manubrium. In some cases it can be seen that the object of this co- 

 ordinated movement is to allow the extremity of the manubrium — i. e. 

 the mouth of the animal — to pick oif a small j)article of food that has 

 become entangled in the marginal tentacles. It is therefore not im- 

 probable that in all cases this is the object of such movements, 

 although in most cases the particle which is caught by the tentacles is 

 too small to be seen with the naked eye. As it is thus no doubt a 

 matter of great importance in the economy of this Medusa that its 

 marginal tentacles should be very sensitive to contact with minute 

 particles, so that a very slight stimulus applied to them should start 

 the co-ordinated movements of localization, it is not surprising that 

 the tentacular rim should present nerve-endings so far sensitive that 

 only by their excitation can the reflex mechanism be thrown into 

 action. But if such is the explanation in this case, it is curious that 

 in Tiaropsis indicans every part of the bell should be equally capable 

 of yielding a stimulus to a precisely similar reflex action. 



On cutting off portions of the margin, and stimulating the bell 

 ahove the portions of the margin removed, the manubrium did not remain 

 passive as it did when the ivhvle margin of the bell was removed ; but 

 it made ineffectual efforts to find the offending body, and in doing so 

 always touched some part of the margin which was still unmutilated. 

 This fact can only be explained by supposing that the stimulus sup- 

 plied to the mutilated part is spread over the bell, and falsely referred 

 by the manubrium to some part of the sensitive — i. e. unmutilated — 

 margin. 



But to complete this account of the localizing movements it is 

 necessary to state one additional fact which, for the sake of clearness, 

 * ' Phil. Trans.,' clxvii. 



