396 D. J. SCOURFIELD ON THE SWIMMING 



had a strong desire to find out the reason why the facts should 

 be as they are. As I have now, I believe, made some progress 

 towards a solution of the problem, it may prove interesting if I 

 place before you what seems to be the explanation of the 

 phenomena in question. In doing so I will follow the steps 

 by which I have been led to my present conclusions, as some of 

 the details of investigation may prove useful to other workers. 



As I did not think it possible that there could be any 

 great difference in the relative positions of the centres of 

 gravity in animals so closely similar to one another, my first 

 theory to accovmt for the difference in the mode of swimming 

 was that it was due to the direction of the stroke of the large 

 antennae. I thought that the normal direction of the stroke 

 in Daphnia and C eriodajohnia would be found to be nearly 

 parallel to the long axis of the body, or, if oblique to this line, 

 then inclined slightly towards the back, whereas in Simocephalus 

 and Scapholeheris] the stroke would be found to be decidedly 

 oblique to the long axis but directed towards the ventral margin. 

 The idea underlying these suppositions was, of course, that the 

 long axis of the body in each case would tend to arrange itself 

 in the line of motion, or in other words parallel to the direction 

 of the stroke. 



To ascertain whether this theory was correct or not, I devised 

 a method of suspending the animals by the top of their heads 

 in such a way that they could use their antennae and all other 

 organs with perfect freedom, and yet not alter their position in 

 the water. This was done by attaching the little creatures by 

 means of a minute drop of some micro- cement to the end of a 

 pin which had been run through a piece of cork made to fit on 

 the top of a small glass tank or trough. (See Fig. 1. The 

 size of the animal is somewhat exaggerated in order to show the 

 method of attachment more clearly). As this method of 

 ■examining living Entomostraca, and also, of course, many other 

 aquatic organisms, seems to offer many possibilities for in- 

 vestigating questions which have not hitherto been within the 

 range of the biologist's " practical politics," I may as well give 

 here such details as will enable any one interested to make 

 similar experiments for himself. 



The first thing is to cut a piece of cork so that the lower 

 half fits between the two plates of a glass trough or similar 



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