The "clocks" timing biological rhythms.
D000818:Animals; D001683:Biological Clocks; D001717:Birds; D003386:Brachyura; D002940:Circadian Rhythm; D003624:Darkness; D008027:Light; D008116:Lizards; D051379:Mice; D009835:Oligochaeta; D051381:Rats; D012474:Salmon