studies on marine Oslracorls 



113 



ming of the Daph- 

 nids and Bosmina. 



In other words these experiments certainly seemed to show that a number of processes 

 in these animals influence more or less the position of the centre of gravity, but, as R. Woltereck 

 points out, this influence was often so slight that it not „geniigt zur Erklarung". 



What are then the functions of these processes? According to R. Woltereck they 3) Organs for stab 

 ought to be looked upon as stability and steering organs, i. e. organs for bringing about ''"^ ""'^ steering. 

 swimming in a straight line. As a matter of fact a similar explanation had been put forward 

 more in passing by G. Burchhardt, 1900, p. 282, in the case of the anterior antennae and the 

 mucrones in Bosmina and the spina in Daphnia; when Woltereck worked out his treatise 

 this statement of G. Burchh.\RDT's was, however, unknown to him (see p. 525). 



Woltereck supports his theory especially by a very thorough analysis of the swimming ft- Woltereck's 

 of the Daphnids and of the genus Bosmina. The result of this analysis is briefly as follows: '"''"'■^* "■ '"* "'*"'■ 



1) In the Daphnids, which take up an almost vertical position when at rest, the The method of swim 

 second antennae, in which the animals are, so to speak, himg up, strike upwards and backwards 

 during swimming. On account of this a contin- 

 uous displacement of the longitudinal axis is 

 produced during swimming. This displacement 

 is illustrated by Woltereck by the adjoined 

 figure XI. 



The following explanation is given of 

 this figure: ,, Instead of the longitudinal axis of 

 the body one may imagine a rod, which is first 

 (phase 1) suspended at the point b and receives 

 a blow against this point from the direction B. 

 The result of this is, first, a progressive move- Fig. 

 ment in the direction B and, secondly, a 

 displacement of the forepart of the body in the 



direction Bj and of the back part of the body in the direction Bi. Phase no. 2: We imagine 

 that the progressive motion is finished and thus the body is now suspended (with its antennae 

 extended) at point b in a position caused by the blow last mentioned. Now gravitation exerts 

 a downward drag in the direction Gi on the heavier back part of the body and an upward pull 

 on the head in the direction Gj. (The dotted line denotes the position of the centre of gravity.) 

 Phase no. 3: This shows the result of this effect of gravitation at the moment when a new 

 thrust is made by the natatory antennae in the direction B. As the axis has not yet reached 

 the position of equilibrium (the centre of gravity is under b), it is affected simultaneously by 

 gravitation and the natatory thrust in directions opposite to each other." 



In Bosmina, which in its normal position is suspended by the second antennae and 

 has the ventral side downwards, the natatory strokes are directed downwards and backwards. 

 In these forms too a continual displacement of the longitudinal axis consequently takes place 

 during swimming. WOLTERECK illustrates this displacement with the following figure XII. 



The following explanation is given of this figure: ,, Phase no. 1: Normal position in swim- 

 ming. The natatory stroke influences the longitudinal axis suspended at point b forward in 



Zoolog. bidrag, Uppsala. Suppl.-Bd. 4.. 



XI. — The (iis|ilacemcnt of the longitudinal axis 

 of I he body of Da/ihnia during swimming. 

 {From R. Woltkreck, 1913.) 



15 



