402 D. J, SCOURFIELD ON THE SWIMMING 



occasionally seen Daphnias turning over and over — i.e. swimming 

 in approximately circular paths — this is certainly not the 

 normal method of progression. There must be other factors at 

 work, then, which prevent these creatures from merely moving 

 round and round, and I do not think we need seek far for the 

 two most important of them. 



In speaking above of the effect of gravity, attention was 

 directed solely to the force exerted upon the body as a whole. 

 •Gravity, however, not only causes the animal to sink, but also, 

 in the case of Daphnia, tends to bring its long axis into a vertical 

 position. This action can be easily detected if a Daphnia be 

 carefully watched when not swimming very rapidly. It will then 

 be seen that during each downward stroke of the antennae the 

 animal assumes a more or less oblique position back upwards, and 

 that during the interval necessary for the raising of the antennae 

 the animal becomes nearly vertical again. In this way the turning 

 effect of the action of the antennae is very largely neutralised — in 

 fact it is altogether checked if the movements of the antennae are 

 sluggish. It must also be borne in mind, in considering this 

 straightening effect of gravity, that, owing to the distance at 

 which it acts being increased, the force will become stronger the 

 more the animal approaches the horizontal. Nevertheless it 

 can readily be supposed that when a Dajyhnia uses its antennae 

 very rapidly the interval of time between each stroke is so 

 much reduced that the straightening effect of gravity does not 

 count for much, and that, therefore, the path taken by the 

 animal should be approximately circular. Now, although it 

 seems quite certain from observation that, as a general rule, the 

 faster a Daphnia travels the more it tends to assume a horizontal 

 position, and the more curved becomes its path, yet we know 

 that the latter does not usually return upon itself. There must 

 be then at least one other factor at work to prevent such 

 approximately circular motion, and I think there can be no 

 doubt that this second neutralis^ing agent is no other than the 

 possession of long shell-spines. 



This idea is supported by several facts. For example, it is 

 just in those species of Daphiiia which are the most active 

 swimmers — i.e. the clear-water forms — that we find the greatest 

 development of the shell-spine, and even the additional possession 

 of a head-spine, or an elongated head. On the other hand, 



