198 University of California Publications in Zoology. [VOL. 4 



I have already stated that such situations are free from swift 

 currents and strong waves. The tidal currents in these extensive 

 flats are unnoticeable on casual observation. With this environ- 

 ment in mind we are ready to consider how the early life-stages 

 of the animal are correlated with these conditions. Period I is 

 spent in the egg-capsule in the security of the burrow. Here all 

 the developmental changes, fitting the animal for the active life 

 of period II, take place, and the beginning is made of the organs 

 of greatest functional importance for period III, viz., muscles and 

 mucus glands. 



Although no accurate determinations were made of the specific 

 gravity of the organism, it was obvious from watching individuals 

 of the three periods that this is least during -period II, as in 

 tornaria. The important characters of period II which are cor- 

 related with the animal's movements are: diminished specific 

 gravity, brought about, no doubt, in part as will be indicated, 

 by the enlargement of the middle and posterior cavities; the 

 climax of efficiency of ciliary activity, in both the ciliary band 

 and apical tuft; and absence of growth, except of muscles and 

 glands, whereby the animal's energy may be more completely 

 used in swimming. 



The activities of the organism at this period are directed, 

 first, to escape from the burrow of the parent, and second, to its 

 suspension in the water during short intervals of time. I say 

 "in suspension," for a little calculation shows that it could not 

 proceed by swimming more than two or three meters during this 

 entire period, and when we consider that the direction of swim- 

 ming is mostly vertical, the actual horizontal progress would be 

 much less. 



Obviously, this part of the animal's life is adapted to make 

 use of the slow tidal currents for dispersal. Judging from 

 the numerous laboratory experiments already described, the 

 swimming cycles would prevent the animal from being carried 

 very far into deep water, for the time of rest at the bottom is 

 often greater than the time of active swimming. Besides we 

 must also take into consideration the fact that the tide moves in 

 a direction away from the deep water as well as toward it. The 

 actual procedure from the burrow of the parent is somewhat as 



