188 GEORGE ItALPH MINES. 



causes slowing of the heart-beats, and if carried far enough, stoppage. 

 Qualitatively these phenomena are alike in the hearts of the ray and 

 of the dogfish. But studied quantitatively the ray heart is found to 

 be much more sensitive to Mg than is the dogfish heart. Thus taking 

 the mean of a number of observations on ray* hearts, the con- 

 centration needed to give a 50 per cent reduction in the rate of 

 beat (calling the rate in the solution with '005 M Mg = 100) was 

 only -009 M. For Scyllium it was "05 M. To stop the ray heart 

 the concentration of Mg • ■ needed was about '02 M, while to stop 

 the heart of Scyllium a concentration usually greater than •! M 

 was required. 



A corresponding difference is found in the sensitiveness of these 

 hearts to the simple trivalent ions. Thus "00001 M Ce * * • readily 

 stops the heart of Eaia, while a concentration approaching ten times 

 this magnitude is needed to produce the same effect on the heart of 

 Scyllium. 



In these respects the heart of the angel fish, RJiinn squatina, was 

 found to resemble that of the dogfish rather than that of the ray. 

 The curve representing its behaviour in the presence of various con- 

 centrations of Mg lies between that plotted for Eaia and that for 

 Scyllium, but much nearer the latter than the former. This is in- 

 teresting, as in the ordinary morphological classification Ehina is 

 placed between the Scylliidae and the Eaiidae. It is said that Ehina 

 resembles rays rather than dogfish in its general habits, yet it is 

 possible that the character indicated by these experiments represents 

 something deeper seated than the details of the responses given by 

 the central nervous system. 



It is at any rate conceivable that by the extension of work along 

 these lines we may learn to express genetic affinity in physico-chemical 

 terms. 



One further point. It will be recollected that the behaviour of a 

 membrane depends not only on its original composition and on the 

 nature of the solution immediately surrounding it, but also to some 

 extent on its previous history, I have found that the blood plasma 

 of Scyllium and of Ehina is of ?+ numerically slightly higher than 



H 

 the plasma of Banc (clavata or hlanda). It is possible that the 

 difference in sensitiveness of the hearts of these animals to the 



* Two species, 11. clavata and 11. hlanda, were used in tliese experiments. No 

 difference could be detected in the behaviour of the hearts of these two species towards 

 electrolytes. Using rays of very different sizes no constant differences were found 

 between the large and the small specimens. 



