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One of a group of metameric hydrocarbons, C5H10, of the ethylene series.

This circlet of gill-lamellae led Cuvier to class the limpets as Cyclobranchiata, and, by erroneous identification of them with the series of metamerically repeated ctenidia of Chiton, to associate the latter mollusc with the former.

In metamerically segmented animals the 322 appearance of the cavities of these sacs is synchronous with, and indeed determines, the appearance of metameric segmentation.

The setae are implanted metamerically in accordance with the metamerism of the body, which consists of a prostomium followed by a number of segments.

In the true Chitonidae there are generally two apertures on each side, and in two species three or four, another instance of the tendency to metameric repetition in the group.