INVEBTEBRATA, CRYPTOGAMIAj MICROSCOPY, ETC. 659 



has been hitherto omitted. If any one of the four radial tubes is 

 irritated, the manubrium will correctly localize the seat of irritation, 

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

 This greater case, so to sjicak, of localizing stimuli in the course of 

 the radial tubes than anywhere else in the umbrelhi except the margin, 

 corresponds with what is found to be the case in T. indicans, and pro- 

 bably has a direct reference to the distribution of the principal nerve- 

 tracts. 



On the whole, therefore, contrasting this case of localization with 

 the closely parallel case presented by T. indicans, it may be said that 

 the two chiefly differ in the fresh-water Medusa, even when un- 

 mutilated, not being able to localize so promptly or so certainly ; and 

 in the localization 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 Medusfe are very intolerant of fresh water, and there- 

 fore, as the fresh- water species must i^resumably 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 

 the effects of fresh water upon the marine species are first dcscribed.t 

 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 vessel, remaiuinfr 

 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 too long. It never survives an exposure of fifteen minutes, but may 

 survive an exposure of ten, and generally survives an exposure of five. 

 But although tlicy thus continue to live for an indefinite time, their 

 vigour is conspicuously and permanently imjiaircd. While in the 

 fresh water irritability persists for a short time after spontaneity has 

 ceased, and the manubrium and tentacles are strongly retracted. 



Turning now to tlic case of the fresh-water 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 scries of two or 

 three tonic spasms separated from one another by an interval of a few 

 seconds. During the next half-minuto the ordinary contractions 

 become progressively weaker, until they fade away into mere twitching 

 convulsions, wliich affect 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, tlitro immediately supervenes a strong and persistent tonic 

 spasm, resembling rigor mortifv, and tlio animal remains motituiless for 

 about twenty minutes. Slight twitching contractions then begin to 

 display themselves, which, however, do not affect the whole bell, but 



* I-ooking to the oiKprmons iiuiiilH;r of nmrino eixscios of Modus.!?, it id nnicli 

 moro ))ri)l)iililo tl.at tin; frL'.-%li-\viiUr sjifcica %verc dorivod from tlnin, than thiitthey 

 wiro ill rived frmii a t'rcHli-wiitrr niici'.itry. 



t For full iiccouut Bco ' I'liil. Tiftu.--.,' clxvii. pp. 711, 71."). 



•J \ '2 



