1908] Davis. — Life-history of DoJichoglossus. 199 



follows : First, a cycle of swimming keeps the animal suspended 

 for a short time, during which it is borne by the tidal current; 

 second, a period of rest on the surface of the mud, until the 

 beginning- of the next cycle. This process being repeated for 

 from twelve to twenty-four hours, the animal would, during this 

 time, be carried a considerable distance from the parental burrow, 

 at least far enough to account for such migrations as are described 

 for San Pedro Bay. (Ritter-Davis '04.) 



During the latter part of this periqd the intervals of rest are 

 longer and longer, until by the time period III is reached the 

 creature is safely located in the region of its future burrow. 



What induces these swimming cycles is by no means clear. 

 Two possibilities are suggested: one of stimulation, the other of 

 rhythmical physiological states. 



In the consideration of stimuli the factor of light must be 

 excluded, since the organism has been shown to be indifferent to 

 light. The only source of stimulation would seem to be contact 

 with the surface of the mud, for temperature, salinity, oxygen 

 content of the water and other conceivable factors are practically 

 constant. 



When Paramoecium comes in contact with a solid or other 

 source of stimulation, a new cycle of movements is instituted 

 (Jennings '04). At first thought it might seem that the free- 

 swimming larva of D. pusiUus behaves in the same way. If, as 

 it touches the mud a new cycle were to begin immediately, the 

 behavior of the two organisms would be essentially the same. But 

 the larva crawls for awhile on the mud, and these crawling 

 periods, even at the height of period II, vary in length. Again, 

 a new cycle is often begun before the animal reaches the mud. 

 This precludes the possibility of contact as a stimulus. It would 

 seem, therefore, impossible to assign any particular stinuilus as 

 a cause for starting the swimming cycle. 



As to these cycles being rhythmical and due to physiological 

 states, the chief objection is that they are not regular either in 

 point of time or extent of movement. In the behavior of Para- 

 moecium and similar organisms where this explanation seems 

 plausible, the animals are adults, and the physiological conditions 

 are fairly constant. In the animal under present consideration, 



