PECULIARITIES OF DAPHNIA AND ITS ALLIES. 403 



species like D. pulex, D. obticsa, D. magna, etc., which are com- 

 paratively sluggish in their normal movements, only exhibit 

 short shell-spines, although even here the young, and the adult 

 males of these forms, which are more energetic than the adult 

 females, invariably possess much longer shell-spines than the 

 latter. Again, in the cases where Daphnias have been seen 

 actually turning over and over, it has always been with com- 

 paratively short-spined species, as, for example, D. magna. 



Coming now to Simocephalus, it is easy to show that the known 

 facts as to the results of gravity, and the direction of the current 

 produced by its antennae, are sufficient to explain its position 

 when swimming, and the dii'ection of its movement. When 

 sinking quietly in the water the position assumed is as nearly 

 as possible as shown in Fig. 5. When swimming normally the 

 position is about as shown in Fig. 6. The increase in the 

 obliquity both of the animal and its path is to be explained, as 

 indicated by the diagrams, in exactly the same way as in the 

 case of Daphnia, the only difference being that, as the centre of 

 gravity in Simocephalus is evidently situated on the dorsal side 

 of the long axis, the animal will swim with its back downwards. 

 Of course the tendency for the position to become more and more 

 oblique, and for the path to become more nearly circular, the 

 faster the animal goes, is present here as well as in the case of 

 Daphnia. And it can be verified by observation that when a 

 Simocephalus does go more than usually fast it really does 

 become more nearly horizontal, even sometimes passing that 

 position and swimming with its head incHned downwards, whilst 

 its path in these circumstances is distinctly curved. As, how- 

 ever, species of Simocephalus are rather heavy creatures, and not 

 very strong swimmers, there seems to be no danger of them ever 

 going so fast as to move round and round, even though they 

 are not provided with the special safeguards against such un- 

 profitable action — viz. shell-spines. 



Except in the introductory paragraphs I have not alluded to 

 Ceriodaphnia and Scapholeheris. This is because, owing to their 

 smaller size, I have not yet made any experiments with them, 

 either to determine the direction of the current produced by the 

 swimming antennae or to ascertain the effects of gravity. I 

 have little doubt, however, that it will be found that the stroke 

 of the antennae in both cases is about the same as in Daphnia 



JouRN. Q. M. C, Series II.— No. 47. 29 



