400 



D. J. SCOURFIELD ON THE SWIMMING 



by the vigorous use of its antennse it managed to make some 

 progress in an obliquely downward direction ; but it was no longer 

 swimming in the manner of a Simocephalus^ but as shown in Fig. 7, 

 with its back upwards. It also clung to the sides of the glass in 

 an inverted position, which is just the opposite to the normal 

 behaviour of animals of this genus. 



The two cases of abnormal swimming just alluded to, taken in 

 connection with what had been already found out about the 

 direction of the stroke, proved conclusively that the main factor 

 in determining the positions of the animals when swimming freely 

 must be the situation of the centre of gravity. Now, although it 

 has not yet been found possible to discover the exact position of 



Figs. 5, 6, and 7. 



The letters have the same significance as in Figs. 2, 3, and 4. 

 h Air-bubble (Fig. 7). 



the centre of gravity in any case, it is possible to find out the net 

 result of the action of gravity. This can be done by watching 

 how the animals sink when they stop using their large antennae. 

 This sounds simple, but in practice it is not so very easy, as 

 Water-fleas are not in the habit of resting on their oars, so 

 to speak, and letting people see what gravity will do to them. 

 Still, as the result of a good many observations, I have no 

 hesitation in saying that the action of gravity upon a Daphnia 

 is to cause it to sink in practically a vertical position, as shown 

 in Fig. 2, whereas the same force causes a Simocephalus to 

 sink in an oblique position back downwards, about as shown 

 in Fig. 5. 



If the foregoing statements about the action of gravity and 



