Bitter-Dai 



-JEnteropneusta. 



189 



Swimming: takes place chiefly, if not wholly, in one direction 

 only, viz., from below upwards. It is accomplished mainly by the 

 flagellate cilia, for when the action of these cilia is inhibited by 

 chloretone the swimming is correspondingly affected. The motion 

 of these cilia is comparatively slow, the girdle as a whole pre- 

 senting to the eye a series of saw-toothed waves, PI. XIX, Fig. 16", 

 similar to those for the rows of cilia on the ciliary plates of Cteno- 

 phora figured by Verworn, p. 571. These waves travel contra- 

 clockwise. Examination of narcotized animals, where the action 

 of the cilia has become slow enough to permit of being observed, 

 shows that their motion with reference to the entire animal is 

 parallel with the body axis. This conclusion is confirmed by the 

 fad that the direction of the water currents set up by the cilia is 

 parallel to the longer axis of the body and toward the posterior. 



Fig. 1. 



The egg probably has nearly the same specific gravity as the late metamor- 

 phosing, or adult stage. With this assumption the life history curve with reference 

 to specific gravity is complete, the dotted portion representing the hypothetical 

 decrease from egg to stage actually determined. 



Tornaria rarely swims vertically upward but almost inva- 

 riably takes a spiral course. The importance of this for economy 

 of energy in overcoming the force of overweight of a plankton 



