114 



TAGR SKOGSnERG 



Fig. XII. — The displacement of the longitudinal 



axis ot the body of Bosmina during swimming. 



(From. R. Woltereck. 1913.) 



the direction B. It also produces a displacement of the axis in the direction Bi and B^. At the 

 siuuc time tlie force of gravity produces an eilect in the same direction, (J, and Go. Phase no. 2: 

 The result of 1. Phase no. 3 (tiot realized): The result of the forces B and (I durinii pliase no. 2." 

 lu other words both in I) a p h n i d s and H o s ni i ii i d s a displacement of the 

 longitudinal axis of the body is produced by the strokes of the natatory antennae. In tlie 



former this displacement is opposed, in the latter 

 assisted, by the force of gravity. 



In the genus Bosmina, provided there are no 

 obstructing forces, an overturning backwards ought 

 consequently to result. If, however, we watch a 

 Bosmina swimming, we see, as R. Woltereck has 

 pointed out, that it goes forward rather swiftly and 

 in a straight line through the water, usually with 

 its ventral side downwards. The factor that makes 

 a straight- lined progressive movement possible and 

 prevents overturning is, according to the same 

 writer, the resistance of the water. WOLTERECK 

 illustrates the resistance of the water with the 

 following figure XIII, 



The following explanation of figure XIII is 

 given: ,,a) The influence of the resistance of the water 

 (\V) on the direction of the motion of a body that is driven forward by a force 

 applied at a point b in the direction B. \\\ and W^ = the deviation of the longitudinal axis 

 due to the greater pressure against the upper half of the body. This results in a continual devi- 

 ation of the direction of motion from B to F. b) This illustrates the same case as fig. a, 

 but a steering surface (S) is developed, which makes the resistance of the water equally great 

 against the upper half and the lower half of the body. Result: The retention of the direction 

 of the force as direction of the motion. If the steering surface is somewhat longer or wider 

 the body i^ made to move forward in the direction of its longitudinal axis (F,)." 



The co-operation of these three forces, the natatory strokes of the antennae, the force 

 of gravity and of the resistance the water, in the swimming of the D a p h n i d s and Bosmina 

 is illustrated by Woltereck by the 

 accompanying figures XIV and XV. 

 This explanation is supported 

 by the above-mentioned amputation 



9/n« other facts in 

 ipport of Wolter- 

 k's opinion, ad- 



incedbythis author, experiments that Woltereck carried 

 out on Daphnids and B o s- 

 m i n i d s. The fact that in many 

 places Bosmina longirostris and B. 

 (coregoni) longispina have short pro- 

 cesses during summer, and long ones 



Fig. XIII. — Diagrams showing the influence of the resistance of 

 the water. (From R. Woltereck, 1913.) 



