192 University of California Publications in Zoology. t v L - 4 



EARLY HABITS AND ACTIVITIES OF D. pUSlllllS. 



Period I. 



As soon as the ciliary band is fully formed, the larva begins 

 active swimming within the egg-capsule. Bateson '84 (p. 211) 

 says: "The larva swims about very rapidly, rubbing the mem- 

 branous shell with its anterior end until it gives way, and the 

 animal escapes." The larva of D. pusillus does not escape as 

 easily as one might suppose from the above statement. As soon 

 as an opening is made, the anterior end is pushed through. The 

 proboscis becomes flattened and the body within the capsule con- 

 stricted. ' The large cilia cease to move, but the small ones are in 

 active motion, producing a slow rotation. The crucial point in 

 the process of hatching is in the passage of the ciliary band. 

 After this portion of the body passes through the opening, the 

 larva is soon free. The average time, for the entire process after 

 the opening is made, as observed in five specimens, was twenty 

 minutes. Within a few minutes after escape from the egg the 

 larva begins active swimming. PJate 7, figs. 17, a, 1}, and c, rep- 

 resents the passage of the larva through the egg case opening. 



Period II. 



As will be noted in connection with the description of period 

 III, period II was not noticed at San Pedro. Bateson '84 (p. 211) 

 says: "On leaving the egg it does not swim at the surface as 

 pelagic larvae do, but creeps about in the mud, burrowing with 

 its proboscis, in the walls of which muscle fibers soon appear, and 

 also propelling itself by means of its ciliated band. If placed in 

 a beaker of water it sinks to the bottom at once. ' ' This descrip- 

 tion, with the exception of one point, applies very well to the 

 third period of D. pusillus, but not to the stage immediately fol- 

 lowing escape from the egg, or period II. The exception above 

 noted is in reference to what Bateson says of "burrowing with 

 its proboscis, in the walls of which muscle fibers soon appear." 

 It is hard to understand how the. animal could burrow with its 

 proboscis before the muscle fibers appear and become active. As 

 a matter of fact in D pusillus when the burrowing stage is reached 

 two sets of muscle fibers have appeared in the proboscis. 



