19 i University of California Publications in Zoology. [VOL. 4 



After several hours the intervals of rest at the bottom grow 

 longer and longer until finally the animal ceases to rise. Light 

 does not seem to have any influence in directing the animal's 

 movements. It swims freely both toward and away from the 

 light. A number of animals were put in a dish from which light 

 was excluded except at a small opening in the cover of the dish. 

 The dish was left undisturbed for five minutes. At the end of 

 this time the animals were found scattered irregularly through 

 the water. This experiment was modified in various ways, and 

 for varying lengths of time, but always with negative results. 



It is during period II that the tuft of apical cilia reaches its 

 fullest development, although it appears in period I. There is 

 reason to suppose that these cilia are of use to the animal at this 

 time. It is impossible to ascertain their exact behavior when the 

 animal is moving rapidly, but when it moves slowly along the 

 bottom of a dish by means of the small cilia with which the body 

 is covered, the tuft swings in a direction opposite that which the 

 animal takes in turning, thus playing the part of a tiller or rudder 

 for steering the creature. It is probable that these large cilia 

 are used in a similar way during the animal's more active move- 

 ments. But the direction is not determined entirely by the apical 

 cilia, for only one part of the ciliary band may beat at a time ; 

 e.g., slow rotation is caused when the cilia of one region on one 

 side beat while the rest are inactive. Furthermore, the rate as 

 well as the direction of swimming is determined by the number 

 of cilia that beat in the ciliary band. The velocity of swimming 

 may be said to vary with the number of cilia in action. When 

 the velocity is increased, as it always is when the animal is rising, 

 this is probably always accomplished by use of additional cilia. 

 In one portion of the swimming cycle the descent is vertical. 

 The cilia of the ciliary band are, at such times, entirely motionless. 



Period HI. 



The few specimens examined at San Pedro in 1902 consisted 

 of unhatched larvae (period I), and free larvae of period III. 

 From the scanty material then at hand I concluded with Bateson 

 that the first period after hatching was a crawling one, and that 

 there was no free swimming period. My observations were de- 



