Slndies on marino Ostrncods 



115 



in winter is also explained in this way. „ln summer these animals swim", as Woltehkck 

 points out, p. 505, „in warm water and when abundantly fed they have swift and energetic 

 natatory strokes; in cold water and with less intensity of assimilation in winter they swim ,at 

 half speed' and consequently need longer steering surfaces." 



Fig. XIV. — The factors of the direction of motion in liic liigli -crested Ili/alodaphnia (Reproduction of fiy:. 28 in 



R. Wolterkck's work of 1913). 



a) „Effect of .self-motion (direction B) and deviation due to gravitation (direction G). The resultant of the parallelogram 



of forces is the direction of motion B. G." 



b) „The total effect of the propelling force, the twisting of the axis, the resistance of the water, the spina and the 

 crest. The dotted arrows show the displacement of the points G 1 — 4, when the head is pressed up about as much 

 as the width of an eye. In this cases the spina acts as a ventral rudder pressing down the head. When, on the other 

 hand, the head is pressed just as much down in the natatory movement (fig. XI: 2), the spina is raised to the same 

 extent and then has a lifting effect on the head (dorsal rudder); it is thus here, on account of its position in the 

 direction of the motion, a typical stability surface. The body always acts aS a ventral rudder because of the different 

 resistances met by the water in the case of the head and the body. The number of the arrows drawn at equal 

 distances from each other only shows the difference of pressure in one plane. .\s a matter of fact the broad body is 



subject to a much stronger pressure than the narrow head." 



w 



Fig. XV. — Factors in the direction of motion of /iosmina (Reproduction of fig. 18 in K. NVoltereck's work of 1913). 



a) „The effect of self-motion (direction B) and deviation owing to gravitation (direction G). The resultant of the 



parallelogram of forces is the ilirection of motion B. G." 



b) „The axis-twisling effect of gravitation at four different points of the body, when the head has been pressed up 

 about as much as the width of the eye = G. g = centre of gravity. \V = the effect on the dorsal and ventral sides 



of the body of the water displaced in swimming forward." 



