PECULIARITIES OF DAPHNIA AND ITS ALLIES. 



399 



compared with the long axis of the body. (See Figs. 2, 3, and 4.) 

 This direction was indeed what I had anticipated from the position 

 of the animals of this genus when swimming, but it left me in the 

 dark as to the reason for the difference in the mode of progression 

 between Daphnia and Slmocephalus. 



The next step forward was due to two lucky accidents. In one 

 case, during an unsuccessful attempt to fix a Daphnia by its head 

 to a pin, in the manner already described, some of the sealing-wax 

 cement was left attached to the animal, about as indicated by w 



Figs. 2, 3, and 4. 



la Long axis of body. 

 g Direction of action of gravity. 

 •ms Direction of current produced by stroke of large antenuje. 

 m^ Direction of tlie forward movement of the animal. 

 10 Sealing wax (Fig. 4). 



The dotted lines have been inserted to complete the parallelo- 

 grams of forces. 



in Fig. 4. When the animal was allowed to go free in the water 

 again, it was observed that it now had a tendency to swim some- 

 what obliquely back downwards, as shown in Fig. 6. In fact, it 

 was imitating the normal method of progression of a Slmocephalus. 

 The other observation was made with a Siniocephcdus. In this 

 instance an air bubble (see Fig. 7, b) had got into the brood-cavity 

 in consequence of the animal having been allowed to remain too 

 long upon a glass slip without water. The creature was now 

 somewhat lighter than water, and its swimming was most curious. 

 It struggled hard against the upward pull of the air bubble, and 



