HABITS, ETC., OF CEREBRATULITS LACTETJS. 150 



way at tlie last moment, and tlie buncli of cells is set free 

 rotating slowly. 



The Pilidium. 

 Habit s. 



Locomotion. — The larva always moves with the aboral 

 surface ahead, as seen in fig. 8. Locomotion is accomplished 

 wholly by the cilia, particularly the band of large cilia 

 around the edge of the oral surface. Their combined action 

 results in a uniform and quite rapid forward motion, and 

 there is also a continuance of the old rotary motion, though 

 the axis of rotation is now horizontal instead of vertical. 



The life of the larva seems to depend on its constant 

 motion, and if one is stopped for any length of time it goes 

 to pieces, the cells being torn asunder by a vigorous contrac- 

 tion of the body muscles. This renders any extended 

 examination of living specimens very difficult. Nevertheless 

 it was determined to examine these larvae alive for the reason 

 that they become beautifully transparent immediately after 

 gastrulation, so that internal development can be followed in 

 detail. 



After repeated trials it was found that the best method of 

 keeping the larvaB quiet was to place them in a drop of salt 

 water under a supported cover-glass, and then paralyse them 

 with cocaine, or better with chloral hydrate. If just the right 

 amount (which must be very small) be used the larva will 

 remain stationary for a long time, with the cilia moving 

 slowly and without any contraction of the muscles. In this 

 condition camera lucida drawings are easily made, and the 

 morphology which follows has been obtained entirely 

 in this way from a study of living larvae. 



Use of Apical Flagella. — In swimming the long 

 flagella are stretched far out ahead, and move round slowly 

 in front of the advancing embryo. So completely are these 

 flagella controlled by the apical muscle that they are capable 



