i8o 



NATURE 



y-iine 24, 18S0 



these movements of localisation continue unimpaired 

 after the margin of the bell has been removed, and will 

 be inefFectually attempted even after the bell is almost en- 

 tirely cut away from its coimections with the manubrium ; 

 in the freshwater Medusa these movements of localisa- 

 tion 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 

 manubriunr to perform its movements of localisation. It 

 is clear that this reason must either be that the margin 

 contains the nerve-centres which preside over these 

 localising movements of the manubrium, or, much more 

 probably, that it contains some peripheral nervous struc- 

 tures which are alone capable of transmitting to the 

 manubrium a stimulus adequate to evoke the movements of 

 localisation. In its unmutilated state this Medusa is at 

 intervals perpetually applying the extremity of its manu- 

 brium 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-ordi- 

 nated movement is to allow the extremity of the manu- 

 brium — i.e., the mouth of the animal — to pick oft a small 

 particle of food that has become entangled in the mar- 

 ginal tentacles. It is therefore not improbable that in 

 nil 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 localisation, it is not surpris- 

 ing 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 

 indicaiis every part of the iDell should be equally capable 

 of yielding a stimulus to a precisely similar reflex 

 action. 



In pursuance of this point I tried the experiment of 

 cutting o^ portions of the margin, and stimulating the 

 bell above /he portions of /he' inai-gin liihich I had removed. 

 I found that in this case the manubrium did not remain 

 passive as it did when the whole margin of the bell was 

 removed ; but that it made ineffectual efforts to find the 

 offending body, and in doing so always touched some 

 part of the margin which was still unmutilated. I can 

 only explain this fact by supposing that the stimulus 

 supplied 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 localising move- 

 ments it is necessary to state one additional fact which, 

 for the sake of clearness, I have hitherto omitted. If 

 any one of the four radial tubes is irritated, the manu- 

 brium will correctly localise the seat of irritation, whether 

 or not the margin of the bell has been previously removed. 

 This greater case, so to speak, of localising stimuli in the 

 course of the radial tubes than anywhere else in the 

 umbrella except the margin, corresponds with what I 

 found to be the case in T. iudicans, and probably has a 

 direct reference to the distribution of the principal nerrc- 

 tracts. 



On the whole, therefore, contrasting this case of locali- 

 sation with the closely parallel case presented by T. 

 indicans, I should say that the two chiefly difter in the 

 freshwater Medusa, even when unmutilated, not being 

 able to localise so promptly or so certainly ; and in the 

 localisation being only performed with reference to the 

 margin and radial tubes, instead of with reference to the 

 whole excitable surface of the animal. 



All marine Medusce are very intolerant of fresh water, 

 and therefore, as the freshwater species must presumably 



have had marine ancestors,' it seemed an interesting 

 question to determine how far this species would prove 

 tolerant of sea water. For the sake of comparison I shall 

 first briefly describe the effects of fresh water upon the 

 marine species.= If a naked-eyed Medusa which is 

 swimming actively in sea water is suddenly transferred 

 to fresh water, it will instantaneously collapse, become 

 motionless, and sink to the bottom of the containing 

 vessel. There it will remain motionless until it dies ; but 

 if it be again transferred to sea water it will recover, 

 provided that its exposure to the fresh water has not been 

 of too long duration. I have never known a naked-eyed 

 Medusa survive an exposure of fifteen minutes ; but they 

 may survive an exposure of ten, and generally survive an 

 exposure of five. But although they thus continue to live 

 for an indefinite time, their vigour is conspicuously and 

 permanently impaired. While in the fresh water irrita- 

 bility persists for a short time after spontaneity has 

 ceased, and the manubrium and tentacles are strongly 

 retracted. 



Turning now to the case of the freshwater species, 

 when first it is dropped into sea water at 85° there is 

 no change in its movements for about fifteen seconds, 

 although the tentacles may be retracted. But then, or a 

 few seconds later, there generally occurs a series of two or 

 three tonic spasms separated from one another by an 

 interval of a few seconds. During the next half minute 

 the ordinary contractions become progressively weaker, 

 until they fade away into mere twitching convulsions, 

 which aft'ect different parts of the bell irregularly. After 

 about a minute from the time of the first immersion all 

 movement ceases, the bell remaining passive in partial 

 systole. There is now no vestige of irritability. If 

 transferred to fresh water after five minutes exposure, 

 there immediately supervenes a strong and persistent 

 tonic spasm, resembling rigor mortis, and the animal 

 remains motionless for about twenty minutes. Slight 

 twitching contractions then begin to display themselves, 

 which, however, do not affect the whole bell, but occur 

 partially. The tonic spasm continues progressively to 

 increase in severity, and gives the outline of the margin a 

 \-ery irregiflar form ; the twitching contractions become 

 weaker and less frequent, till at last they altogether die 

 away. Irritability, however, still continues for a time — a 

 nip with the forceps being followed by a bout of rhyth- 

 mical contractions. Death occurs in several hours in 

 strong and irregidar systole. 



I f the exposure to sea water has only lasted two minutes, 

 a similar series of phenomena are presented, except that 

 the spontaneous twitching movements supervene in much 

 less time than twenty minutes. But an exposure of even 

 one minute may determine a fatal result a few hours after 

 the Medusa has been restored to fresh water. 



Contact with sea water causes an opalescence and 

 essential disintegration of the tissues, which precisely 

 resemble the eftects of fresh water upon the marine 

 Medusa:. When immersed in sea water this Medusa 

 floats upon the surface, owing to its smaller specific 

 gravity. 



In diluted sea water (50 per cent.) the preliminary tonic 

 spasms do not occur, but all the other phases are the 

 saine, though extended through a longer period. In sea 

 water still more diluted (i in 4 or 6) there is a gradual 

 loss of spontaneity, till all movement ceases, shortly after 

 which irritability also disappears ; manubrium and ten- 

 tacles expanded. After an hour's continued exposure 

 intense rigor mortis slowly and progressively develops 

 itself, so that at last the bell has shrivelled almost 

 to nothing. An exposure of a few minutes to this 

 strength places the animal past recovery when restored 



' Looking to the enormous number of marine species of Medusa, it si 

 much more probable that the fre5hw.ater species were derived from them, 

 than that they were derived from a freshwater ancestrj-. 



= For full account, see Phil. Trans., vol. clxvii., pp. 744-745- 



