An explanation of the apparent reversal of the circadian migration by Hymenolepis diminuta (Cestoda) in the rat.

Hymenolepis diminuta exhibits 2 concurrent migrations: an age-dependent, forward migration and a circadian migration. In experiments where the age of worms was assumed to be uniform throughout the test-day, 2 patterns (7-day-old and 16-day-old) of circadian migration were seen and an apparent reversal in circadian pattern suggested. In experiments where the age of worms became progressively older during the test-day, only the 16-day-old pattern was seen and no reversal was observed. The 7-day-old pattern and hence the apparent reversal in circadian migration is postulated to be an artifact of the method whereby both the age and size of worms were presumed to be uniform throughout the test-day. The 7-day-old pattern results from comparing worms of unequal size and thus at different positions in their forward migration. Data on the daily variation of stomach contents of rats and the results from protein determinations of 6-day-old worms support this hypothesis. Therefore H. diminuta is believed to exhibit only the 16-day-old pattern of circadian migration: an anterior migration between 12 midnight and 6 AM, and a posterior migration between 12 noon and 6 PM.