190 University of California Publications. [zoology, 



organism may be illustrated by comparison with the upward flight 

 of birds. The problem for swimming aud flying is essentially the 

 same so far as this element is concerned. 



Concerning hovering in bird flight, LeConte, '94, p. 746, 

 says: "In maintaining the body in the same position, as in 

 hovering, the air gives way under each stroke of the wing, 

 creating a downward current, thus diminishing the effectiveness 

 of the down stroke and increasing the loss in recovery or the up 

 stroke. In progressive flight, on the contrary, and more and 

 more as progress is more rapid, every phase of the down stroke 

 is on new air. . . . But if it is difficult, on this principle, 

 to maintain one position as in hovering, it is evidently still more 

 difficult on this principle to raise the body directly upward." 

 Substituting water for air and cilia for wings, this principle 

 applies to swimming aud shows the advantage of a spiral course 

 over one directly upward. New areas of water are constantly 

 being put into motion by the cilia in the spiral progression of 

 the animal without the retarding influence of downward currents. 



The spiral course may be accounted for as follows: As the 

 animal moves forward in the direction of its axis each cilinm 

 pushes sidewise against the water, throwing the body in the 

 opposite direction. This action passes around the animal in 

 successive sidewise pushes corresponding to the crests of the 

 ciliary waves, PI. XIX, Fig. 16-'. If these crests are in pairs 

 directly opposite each other, the sidewise push on one side will 

 be neutralized by a counter sidewise push on the opposite side 

 and the animal will thus maintain its axial direction. On the 

 other hand, if the crests are not in pairs directly opposite each 

 other, a spiral instead of an axial course will result. The latter 

 explanation applies to the spiral course of tornaria, Fig. 16. 



Besides the body movements just described, the animal slowly 

 rotates around its longer axis. The origin of this action is 

 not clear. Two possibilities suggest themselves which may 



a unt for it: The cilia may not strike exactly parallel with the 



body axis-, or the depressions between successive waves may 

 run obliquely across the ciliary girdle so as to form a turbine-like 

 groove. It was impossible to determine by observation which of 

 these, if either, is used. The fact that the rotary motion is slow 



