Figure 6. Average body length of N. vectensis increases in darkness.

A) Average body length of 9 N. vectensis over a 24 hr period. Animals were housed in a 12 hr: 12 hr LD photoperiod (shaded area indicates period of lights off). Body length was significantly greater during early night as compared to time points during the day (*Newman-Keuls; p<0.05). Bars represent standard error. B) Images of an individual animal housed in a petri dish taken at various time points throughout the day. Animals remain constricted during the day and increase in length significantly during early night to an elongated, more active state. Oral and aboral ends of the animal are indicated by arrows. While peak length occurs just after lights off, animals begin to anticipate the onset of darkness by initiating elongation several hours earlier. This anticipatory behavior is characteristic of endogenous clock control.